I thought transformations were dropped because the devs liked standalone characters more

[Crazy Hand 2001]

Which is why Zelda, Shiek and the Samuses remained split up.

so why would Pyra and Mythra be transformations?

 

[Wario Wario Wario]

Pyra and Mythra aren't really seperate characters as far as I can tell - just a few stat and animation changes with unique models.

 

[Perkilator]

Pyra and Mythra aren't really seperate characters as far as I can tell - just a few stat and animation changes with unique models.

The Ultimate website lists Pyra and Mythra as #79 and #80, respectively. Also, in the trailer, they are very clearly shown to have different specials and even Final Smashes.

So yes, despite being transformation characters being jammed in a single slot, they are indeed separate fighters.

 

[Handy Man]

For Smash 4, transforming characters were removed because the 3DS’s hardware limitations couldn’t handle transforming characters, and Sakurai wanted the Wii U version to be consistent with the 3DS version. This is why Zelda and Sheik are seperate characters in Smash 4, as well as why the Pokémon Trainer was cut and only Charizard was playable. In Smash Ultimate, the Switch was capable of processing transformable characters once again, so that’s why Pokémon Trainer was able to return in full. There have been no comments whatsoever about “the developers preferring standalone characters”.

 

[AnEasterEgg]

I'd imagine another reason they'd want to separate the past transformation fighters is that were generally a case of "one is better 99% of the time". With Pyra and Mythra having a really fast switch speed and similar standard moves, I think they're trying to fix that here and make both of them worth using.

And of course it's also just faithful to the characters to have them both. Although ironically, they essentially had the same problem in Xenoblade 2, with Mythra being almost always more powerful than Pyra.

 

[RetrogamerMax]

Well, that was just fan assumption. And like what Handy Man said:

For Smash 4, transforming characters were removed because the 3DS’s hardware limitations couldn’t handle transforming characters, and Sakurai wanted the Wii U version to be consistent with the 3DS version. This is why Zelda and Sheik are seperate characters in Smash 4, as well as why the Pokémon Trainer was cut and only Charizard was playable. In Smash Ultimate, the Switch was capable of processing transformable characters once again, so that’s why Pokémon Trainer was able to return in full. There have been no comments whatsoever about “the developers preferring standalone characters”.

 

[Starlight Liger]

I think her position as a paid DLC character justifies her being a switchable character.

 

[Bobthealligator]

For Smash 4, transforming characters were removed because the 3DS’s hardware limitations couldn’t handle transforming characters, and Sakurai wanted the Wii U version to be consistent with the 3DS version. This is why Zelda and Sheik are seperate characters in Smash 4, as well as why the Pokémon Trainer was cut and only Charizard was playable. In Smash Ultimate, the Switch was capable of processing transformable characters once again, so that’s why Pokémon Trainer was able to return in full. There have been no comments whatsoever about “the developers preferring standalone characters”.

Actually Sakurai did say both, I feel like the thing about learning multiple movesets was a "Probably for the best" thing rather than "We did this because".

 

[Bobert]

Pokemon Trainer and arguably Joker are transformation characters, but ZSS and Zelda were held back moveset wise when they were apart of another character. ZSS solely depended on switching via final smash so there was no point to them being tied to a slot anyway and simply wasted ZSS having a unique FS. Zelda especially was hurt because she lacked decent approaching and zoning tools besides literally switching to Sheik and using needles for zoning, and literally just running at the opponent with her superior speed and aerials for approaching. Now she has a decent zoning/approaching tool in down special, and Sheik can no longer steal her thunder. There's also the fact that they wanted to use iterations from different games to represent multiple games in the series, and LttP Zelda isn't exactly known for transforming into a ninja. Albeit she doesn't summon the armor from Spirit Tracks either but...that's easier to look past than Zelda turning into...a different Zelda from a different era entirely, except she's in a ninja cosplay.

Pyra/Mythra work better than Sheik/Zelda from the get go because they are effectively the same character with stat changes and different specials, with said differences also being much more well balanced. One is much better at racking up damage at low %'s with fast combos, dealing with faster characters, being generally hard to catch due to being faster in all regards as well as Foresight, and having a better neutral in general. While the other is better at getting raw kills and just being much scarier when you're in disadvantage in general, such as offstage or above her. She also should excel at keeping the damage on even after characters start to go to far for Mythra to get any combos. Similar to how Charizard and Ivysaur can continue to rack up damage when Squirtle can't anymore. Compare this to Sheik/Zelda, where Sheik excels at approaching, zoning, combos, juggling, catching landings, getting out of disadvantage, generally being one of the most terrifying characters at close range...whereas the only thing Zelda was better at was getting possibly getting an early kill off of a read even though Sheik still had an easier time fishing for kills due to her extremely oppressive neutral. Nowadays, Zelda is generally a much scarier character in general largely thanks to having a frankly ridiculous down special that actual supports her moveset and gameplan amazingly.

In summary:
We still have transformation characters with PT being a literal character change, and Joker since Arsene changes his entire moveset.

Samus and ZSS weren't true transformation characters and required a final smash to change between the two in-game, or a button combination mid match that changes you into one but locks you out of the other.

Sheik/Zelda was pointless because Sheik is just better and takes up a slot Zelda could use to help her gameplay. Also there's the fact they wanted to represent versions of the characters from other Zelda games, and having LttP Zelda turn into OoT Zelda in a ninja costume would be very strange. It would be like having OoT Ganondorf turn into Calamity Ganon from BoTW or Beast Ganon from TP.