@atomicpoet @pixelfed Three possible reasons:

Pixelfed hasn't had a publicly available mobile app outside its small beta testing pool.
It's an Instagram replacement - a significant number of people here are more likely interested in predominantly text-based content than predominantly image-based content.
Instagram is a mobile app and it's users expect a mobile app experience, so not having a Pixelfed app is likely restricting its appeal.

(NB I'm trying it out)