Could also have something to do with Meteor being a solution for both the client and the server. It's not natural to take Meteor for only the server, or only the client.
If you're used to running Rails/Django along with Angular and SQL database, to switch to Meteor, you basically have to switch out everything, and that's probably scary for most people.
I realise you can use Angular instead of Blaze, and you can start using SQL, through packages, but these aren't yet supported by by MDG.
The scariest part is probably switching backends. If you have a team of strong Ruby developers, it'd take a lot to switch your team to this new platform.
If you look at big apps built using each platform, for Rails I get:
For Django I get: http://codecondo.com/popular-websites-django/
Those apps inspire confidence. But what do I get for Meteor? Nothing of the magnitude of the sites posted above. I'm guessing this will change. Meteor is still very young.
One other area that seems to be subpar is testing. Velocity just doesn't seem to be up to it (although I haven't played with it in a while, so maybe things have improved a bit).
But despite all this I agree that Meteor is awesome and I hope it continues to grow.