Because its brand new, written by pretty much one person (I'll give you a hint, it's me), and also sqlite barely supports migrations itself (http://sqlite.org/faq.html#q11). You're trolling pretty hard in this thread, maybe you should let other toolsets besides django have a chance.
I'm sure your code is fine, but you can't possibly argue that it is on par with a major component of a major framework and be taken seriously.
Which is fair enough IMO, but it's not really convincing me that Flask is a better way to go.
https://speakerdeck.com/u/alex/p/why-i-hate-the-django-orm
Yet the developmental resources SQLAlchemy and Alembic are a tiny fraction of what Django has. It's the tremendous gravitational pull of Django and the habits of its community writing things hardcoded to Django that suck the life out of other projects in the Python community.
And yes, I know you wrote it. You wrote SQLAlchemy too.