I was an early RethinkDB user - we probably had one of the largest RethinkDB clusters in production by 2013 at an ad network.
I've gotten to know some early folks at MongoDB pretty well over the last year and RethinkDB has come up a few times. I think the answer comes down to a pretty boring one: MongoDB just got there a bit faster, raised a large sum of money, advanced their product greatly with the purchase of WiredTiger[0] and marketed the hell out of the space. The differentiation point for another document database in 2013-2016 was really limited.
The product they built with such a modest amount of funding was nothing short of amazing. I'll always have a special place in my heart for RethinkDB
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