I suppose why some productions suffer from bad animation is either the character design or the aesthetic in general doesn’t lend itself to another medium well. That’s like wondering why some superhero costumes look awful in live action, there’s an aspect that seems either too hard to believe or use in whatever means and manner.
So it’s sometimes changed into something that adapts more easily to its adopted medium. Otherwise had it stayed faithful, it wouldn’t look that good elsewhere. There are situations that demand greater fidelity in respect to both the author and the target audience so whenever it does stray away it gets backlash.
It’s really a situation where it looks good in one medium but gives others problems in another as if it was originally made for the former and that might explain the need for redesigns in those cases.