Why do they make the animations so low fps?

[aK]

All these anime type fighting games look horrible, the animations are so low fps on purpose for frame data reasons, but that just makes it look ugly. Such nice art design and backgrounds running at 60fps, only to have the characters so weakly animated rather than have nice fluid movement.

This coupled with the fact that everything is so fast paced plus the low animation frame rates it's easy to see why people can't get hooked.

Why do they insist such low frames for the most important part of the game, the characters?

[Red]

You said it yourself: frame data reasons. However, I wouldn't say characters are "the most important part of the game" even though they are definitely important, because they're part of a whole.

Guilty Gear STRIVE will have more animation frames, but I still think it's good to have clear key frames in Xrd.

[sega dreamcast]

To make it look like a 2D anime fighter

[Slaskburk]

As you mention, a lot of it ties down to making a playable game where controlling your character feels responsive enough. One other major aspect is that the developers intentionally reduced the number of frames for certain moves and animation in order to invoke an "authentic" anime aesthetic, as in making the game resemble a traditional 2D game while reaping all the benefits of using a 3D engine with models.

There is a really good GDC panel delivered by ASW's technical artist Junya C Motomura where he he explains their techniques and reasoning for choosing that approach.
https://youtu.be/yhGjCzxJV3E?t=1906 (31:46)
Here's the part where he talks about limited animation, and about 2 minutes in from the timestamp he shows a comparison between the limited and full animation of a move.

The next game in the franchise might be more up your alley as they're moving away from limited animations.

[Suntorias]

Here's a later, shorter video detailing the why on a broader scale, as opposed to the technical presentation above:

https://www.youtube.com/watch?v=kZsboyfs-L4

It shortly mentions how ArcSys also uses 3D models as a base to make their sprite production more efficient in their earlier, sprite based games, and it's mentioned both in the GDC presentation AND the Skullgirls GDC presentation - arguably the best classic cartoon style animation fighting game - (https://www.youtube.com/watch?v=Mw0h9WmBlsw) that using proper interpolation - aka "proper continuous movement" - made everything look uncanny, so they manually had to REMOVE frames that had already existed to make the animations feel right.

There's even a comparison of this in the GDC video in the comment above, where you see what a specific move would look like with continuous animation. It looks horrible. You can see this at 33:55 in the GDC video.