I think OpenToonz is relatively well known, but not really what most people mean when they say open source 2D animation software. OpenToonz is really set up for traditional animation, which is incredibly time consuming. The average hobbyist wants an open source Flash, something that they can use to produce (admittedly lower quality) animation in their evenings after work. OpenToonz *can* be used that way, but that's not really what it was made for.
Oh my I thought I had mentioned synfig in my original comment but I guess not. Synfig is sooo close to being what I think most people want from a FLOSS animation program, but right now it's far too buggy for any serious use. I can't wait for it to be ready, and to get more popular!
@rdh @arisunz *jumps in*
From looking at the way how people talk about Krita's animation functionality, I also think people don't understand the workflow that opentoonz was designed for. They usually complain it is too hard to use.
Especially the original Ghibli version of toonz, which had everything not useful for traditional animation turned off. Like, I've seen people not wanting to believe Japanese anime studios still have traditional pencil workflows as part of the whole.