Don't get me wrong, Flask seems to have a lot of mind-share, it just seems like a lot of people would be interested in working with Python 3.
Personally I am in no hurry to switch to Python 3 and Flask "just works" and does everything I need in a, at least for me, sane way.
[1] http://lucumr.pocoo.org/2013/5/21/porting-to-python-3-redux/
Edit: they = Flask and some dependencies
When I was starting, my first thought (like most people) was to work with Django, but what drove me crazy was that my project started with a bunch of files and code that I didn't write. I want all of that to be abstracted away via plugins and modules, and Flask does that really well.
I'm not sure how that compares to Bottle or other Python frameworks, but I feel like Flask suits me perfectly.
Regarding Python 3 support, a port is in progress so as Python 3 gains traction I think Flask will emerge as part of that transition, but for now Python 2 works well for most people.
I haven't done much Python web development before, but I have a fair amount of experience with PHP and Ruby, so I found stitching together something useful with Bottle and a number of other packages was pretty low-friction other than porting a half-dozen different packages to Python 3. Unfortunately looking at flask it seemed the work to port it along with jinja2 and werkzeug was too much of a barrier to entry.
[1] http://lucumr.pocoo.org/2013/5/21/porting-to-python-3-redux/
I used and played with bottle first, and seriously thought about it for implementing my restful-ish APIs.