The company got bankrupted and they set up an org to fund its development. (Sagemath moved to Postgresql from RethinkDB after the company got sold. The guy behind sage said it was better in his blog about the switch)
Incidentally, http://www.lmfdb.org uses MongoDB (since maybe 2011) pretty heavily, as a result of me convincing them to do so after I read too much about MongoDB back then. It’s been painful for them, but it does still work well...