@dansup @pixelfed
Mastodon for short-ish updates.
Pixelfed for pictures.
Peertube for videos.

I couldn't tell you what Misskey is.

I don't know about everyone else, but if I am anything to go by, the reason may simply be that Pixelfed is good at telling people what it is and has a name that makes it easy ro remember (Pixelfed for pixels, Peertube like Youtube - and Mastodon is the big one we meet first, so it doesn't need an obvious name quite as much).