@molly0xfff
Because people who follow that hashtag are going to see every post in the long thread, instead of just the first (or last) one (which they can click on to see it all if desired).
"people can mute hashtags" - news to me. You would only need to do that for an account you follow, and if an account you follow is posting lots of long threads then you would just unfollow them to begin with (as a couple of us here have already said we did), and follow the hashtags you do want to see.