I think users (and myself included) were just used to doing it the old way. I’d rather quote everything in the post and remove whatever I don’t want to quote. Both on desktop and mobile.
It’s a nifty feature to select and quote just a piece of the text but I don’t think this is the default usage at all. Just from old habit from other systems perhaps.
(Personally I don’t like popups on select but maybe that’s on me)