Because MongoDB is a document store and you really only need a key/value store for this.
Also because MongoDB started life preferring speed vs durability, which is a bad look for a database. They may have fixed that now, but I cannot forgive them for that.
"The best tool is the one I know".
Except when it's not. :)
I don't compare HTML vs. C here, obviously I am talking about databases. All personal feelings aside, there is no technical reason to NOT use MongoDB as a key value store if one has experience with it, because after release comes maintenance and learning a new tool while fixing production problems doesn't make sense to me.
I remember reading on here at one time that under some scenarios your data doesn't come out the way you put it in, or something to that effect.