Because your programmers don't know SQL and don't want to. In an ideal world of perfectly spherical programmers, choosing JavaScript would be completely orthogonal to database choice, but in reality one common symptom of JavaScriptitis is considering SQL an old relic that's too much of a pain in the ass to learn. Thankfully, MongoDB's query language is JavaScript and its result sets are JSON objects.

To be fair, this sort of feeling toward SQL is not found uniquely among JavaScript programmers; one major reason why we have ORM hell is because a previous generation of programmers did not want to leave the warm cocoon of treating data as objects in Java.