I can't speak to everyone, but my artist friends all moved to Instagram because it's a better place to post photos/videos, and the comment system is preferable.
As an artist, yeah. I have a pretty big tumblr following that I've built up over the years, but I also prefer instagram even though I've only had it for a little over a year and haven't built up nearly as big of a base. I think for me it's that instagram feels more for the everyman, in a way. Plenty of grandparents out there with instagrams full of their grandkids, y'know? You reach a much broader audience.