Threads automatically adds all your instagram follows so for creators they don't have to try and rebuild their audience on a new platform where as for users they don't have to find all their friends/family/and creators on a new platform, its just there. There is also the ease of signup, if you have an instagram account you just log in, if you don't it's really simple. For Mastodon there's different servers to pick and then you have to find your favorite creators again and it's another login to remember.

TLDR people are lazy so go with the option that is easiest for them