Well (1) they were right at the limit of where the horse-steppe package could go and history shows that adapting to new packages is actually pretty hard to do. (2) I think by the time they arrived, the Chukchi were already herding reindeer. And the Chukchi were fairly capable war-fighters, so they formed a pretty decent barrier.
fasquardon
Yeah, which is why I'd be skeptical the Indo-Europeans could do what the Yakuts didn't.