The entire sell of MongoDB in the Node community seems to be "it speaks JavaScript and you can store objects in it". Which of course fails to take into account any of the strengths and weaknesses of MongoDB (in the latter case especially the lack of cross-collection transactions or fast joins -- which in my experience account for 90% of the problems Node developers face with MongoDB).

The main reason MongoDB is popular in the Node community is that both reached peak hype around the same time. So it made perfect sense to bundle both of the "hottest" technologies together. Except by the point Angular 1 reached peak hype MongoDB was already facing criticism, so MEAN mostly seems to bask in its afterglow.

If you had asked me last month, my prediction would have been a RethinkDB+Node+React (or RethinkDB+Node+Angular2) stack popping up to challenge MEAN, though the RethinkDB+Node bit seems to have already been addressed by Rethink's own Horizon.