Ariel is the most popular princess and so all toy companies who get her license make a profusion of Ariel merchandise. And, mermaid dolls are also immensely popular. So, it just makes $$$ sense for Hasbro to release a set of Mermaid sister dolls. Arista is always included because she's blonde, and then one brunette is included (you'll see they are different here---Mattel chose Alana, and Hasbro chose Aquata) to make a 'blond, redhead, brunette' pack. Even Jakks Pacific made a set of 3 Mermaid Sisters Petite dolls (they chose Attina, Ariel and Aquata for their 3 pack set). I think they choose 3 because it 'looks' like a lot of dolls but the price can still be kept low for budget friendly doll sets.
Then Ariel always needs a 'mermaid' and 'human' doll pack, and since she's a mermaid, now that they are able to make 'swimmy' dolls, they always make a 'swimming' mermaid Ariel for kids to play with in the bath or pool.
It's typical dollfare for the Little Mermaid. Tyco, who owned the right originally before Mattel, did the same thing.
They had mermaid sisters/friends (Arista in several outfits), tons of Ariel 'mermaid' and 'human' dress doll sets, wedding dolls, and even a 'Holiday' doll, which Mattel also did two or three times after Tyco did. I don't think Mattel copied Tyco but they saw it was a good gimmick or product, so each of the companies probably are just trying to cash in on 'what works'.
So I don't necessarily think it's copying but just knowing 'that sold well, let's continue the line' sort of thing.
And Hasbro is coming up with their own original stuff too, like 'paint your own' dress dolls (where water makes a design show up), and bubble princess Ariel (blow bubbles through a wand with her), an Ariel and vanity doll set, musical Ariel, transforming Ariel, Ribbon Salon Ariel, Color Change Ariel---the list is going on and on! Too many Ariel dolls. I know Mattel had several of these types, too, (Musical Ariel for one) but it just means that type of doll/toy 'works' with kids/collectors and so the new company wants to make their own style of it as well, I think.
A lot of people are comparing Hasbro unfavorably to Mattel dolls but we need to remember to compare the Hasbro to the
later Mattel dolls---where they had plastic molded tops, horrid front side only gowns and bad shorter hair. We can't compare $10-15 Hasbro dolls to $20-25 Mattel dolls. It's like apples and oranges. We have to compare $10-15 Hasbro modern princess dolls to Mattel's $12-15 modern princess dolls.
Now the swimming dolls are comparable as both retailed for around $18-20 each. Same with the Mermaid Sisters Set. Probably both are/were around $30-35 retail.
Mattel's early dolls were good (the faces were not great) in that they had real full dresses, and long flowing hair. But then they went downhill....fast. In 2015 ALL the Mattel Princess dolls were cheap, hard, non jointed, non poseable, molded body bodices and flat skirts with short gluey hair. Many even had NON REMOVABLE shoes!!!! This is why Disney pulled the rights from Mattel, because they were making a travesty of the line. But you can't really blame Mattel much because the rights to the princesses cost a lot and they had to cut corners where they could to keep the dolls around $15 each so the public would still buy.
Hasbro is in the same boat, but they are trying to keep the dolls at $10-12 each because people just don't want to pay more for 'toys' nowadays. (I think the dollar stores and generic dolls have gotten folks to think dolls should be only a few bucks.) So Hasbro has to try to make appealing dolls at low prices. Their first editions were okay with full removable dresses and shoes and long hair. As time goes on, I'm sure we'll be seeing more and more of the 'molded' bodice syndrome but for now, they're still doing full dresses for most of the dolls.
I have a 'Winter Dreams' Hasbro Elsa doll from 2017/2018 which is a more 'collector' line doll, by Hasbro, and she is lovely, in a gorgeous box/display, fully jointed and she even has real eyelashes and a beautiful iridescent quality gown! So Hasbro can 'step up' and make collector quality dolls, too, like Mattel did back in the 1990s/early 2000s. But she retailed for around $30, much like the Mattel 'collector' dolls did, too 20 years ago+.
For myself, I like to see how each company attempts to recreate a character or gown. For playline, most fail miserably, actually in terms of the gowns, but once in a while, we get a winner!
Plus, like DollyKim-san said, the companies try to emulate the animated character's look as much as possible, but in their 'style'. (Mattel had small heads, while Hasbro has big anime like heads, though since 2018, they've been starting to make 'medium' heads. I prefer the big ones since they're so different and chibi! The medium ones are just 'meh' dolls because they aren't accurate but 'try' to be. When the Hasbro dolls came out in 2015/2016, they had huge heads which were adorable and so unique. It made them 'special' in a way that they weren't trying to compete with Disney Store dolls which are accurate to the characters, but that they were making dolls in a whole different style! I loved them!)
So, I think we see so much similar Ariel merchandise because she's the most popular princess. (I never knew that before! I thought she was one of the most popular but not THE most! I thought that honor belonged to Cinderella! But now, according to most fans/polls, it seems it's: 1-Ariel, 2-Belle, 3-Elsa and then 4-Cinderella and 5-Aurora! Wow!)
I just hope that Hasbro doesn't forget the 'lesser known' princesses like Pocahontas, Mulan, Tiana, Merida and includes them in new merchandise, too.
I remember Mattel shamefully ignored Pocahontas entirely after 1998/1999. Since then, they NEVER released one Pocahontas doll with a new sculpt, like they did EVERY OTHER princess.
I have a Mattel Disney Princess collection and use their latest Princess sculpts which looked most like the Disney gals but had medium sized heads, not small like their older sculpts did. (The small non accurate ones that looked more like Barbie doll faces) Anyway, they NEVER made a matching Pocahontas and I was at a loss as to what to do! I couldn't use the old Mattel sculpt because it looked like she had a shrunken head compared to the other princesses! So a couple years ago, Disney Store made a new Pocahontas sculpt and I am now using that in my 'Mattel' Princess collection. It's different than the other DS sculpt and at least it matches, sizewise, the other Mattel Princesses.
So I'm just hoping that Hasbro doesn't go down the same route, ignoring the less popular girls. So far, so good. They remade everyone with the 'medium' sized heads now and even came out with a 'Pocahontas and Meeko' pack and a 'Mulan Transforming' pack. Yay!!!
Anyway, sorry I went on for so long!! I just love the Disney Princesses and their dolls!!! (I collect all 3 types ---Disney Store, Mattel and Hasbro---it's great to see the differences between them!) I love all 3 sets of my Disney princess dolls---Disney Store, Mattel and Hasbro. I think they each bring something new and different to the table and all are great representations of the princesses (if you choose the best of each brand to collect.)
Thanks for asking a fun deep question!!!!