I agree with the comment about distributed transactions - basically, Mongo had the first mover advantage and Rethink (albeit a great platform and easy to setup

I agree with the comment about distributed transactions - basically, Mongo had the first mover advantage and Rethink (albeit a great platform and easy to setup / administer) needed a big differentiator. That's actually why I'm using cockroach right now, they picked a great problem to solve (cross dc / cross continent transactions) that enables a ton of developer use cases.