That seems like a fair comment, I wonder why you're being downvoted?

Because it's neither constructive nor fair. The comment is merely "true".

Yes, the released components do not cover a use case that they were not designed or intended to cover.

Yes, if your use case is exactly that, you will find these components less useful as you'll have to modify them.

Yet all of that is trite, and completely unfair criticism to wager against someone on the very first day of their components release.

if enough people call out the lack of accessibility in UI libraries, it brings that requirement into the cultural norm.

The commenter is merely trying to push this into the cultural norm. I concur with such sentiments.

I think there are better times to do this than the initial version 0 release of an open source production.

Taking the chance to "call out" its "lack", is essentially shaming people for doing social good (open source), and not going far enough in someone's opinion.

You can think what you want. The people who downvoted me for simply asking why the parent was downvoted can think what they want. The mods can think what they want. It doesn't justify anyone for downvoting the parent's comment which was his view or mine for calling out the unjustified downvoting. This is where the mods should step in, find out who was downvoting both the parent and me; and remove their ability to downvote for a time.

Tacking on accessibility at the end is very difficult (most of the time it's quicker to rewrite). By not considering it from the beginning it means these components will never be usable by people with access needs.

Also it is possible to call out a critical oversight in the design of a product while not shaming them for releasing it.