Honestly, as someone else said, US based company, US based standards. Plus, youtube is really designed to show content designed for youtube, not old media from other platforms.

Even the BBC uploads old 576 content to iplayer as 720.
Because if you’re going to do the de-interlace etc before uploading, you might as well put it in a format that everyone is expecting.

It’s weird though, I swear I had a memory of 576 content being an option at some point…Maybe it was just some content being 50Hz.

Because America and NTSC use instead of PAL would be my guess.

Every additional resolution they support is another copy of every single video in their library. 576p is already a European standard not an American one so that is why they probably never supported it to begin with and they’ll probably never supported it in the future.