Of course there are the usual reasons that it is open source, most of the standard libraries are in Factor itself, and that it has a really cool development environment. But almost every popular language these days has all that stuff.
The real reason I think factor is cool is because it still isn't near complete!
It's small and so the sense of community is much stronger, you can be the first to do things in Factor, when they've already been done elsewhere, and anyone with a decent education and experience could easily grok the theory too.
I would say Factor's basic constructs are even smaller than Python's. Just knowing a few shuffle words and USING:, defining words and quotations, gives you almost everything you need to know.
Everything else is just built on top of this.
At present Factor users are probably just in 2 digits. And there are no products (that I know of). Which means you could probably restructure the whole language without affecting anyone. This allows Factor to improve by rewrite, rather than by add-on.
Writing libraries for a new language means doing stuff that most have probably never done. So you have to learn about Unicode, or just traverse strings recursively (?!?) like I had to do in brainfacktor. ( Could it be better? )
Basically by keeping the core small, keeping a single stack and enforcing recursion by not having loops, Factor really forces iterative programmers to go stack based/functional. And that itself is a lot of fun, even though it is hard.
Reading about all the developers coding the standard libraries is also very informative, as is looking through the Factor sources.
And it would be fun to look at the interpreter, though I've never done it.
So that was my attempt at some evangelism :D