Mongo had better marketing. They sponsored hackathons, they had great out-of-the-box UX, they filled their homepage with logos of both 'big enterprise' and 'hip startup' companies using it, and were able to present their product in a way to capitalize on the rise of Node.js and the 'NoSQL' keyword hype.

In fact, the 'MEAN stack' was coined by a MongoDB developer on his blog [1], a post which is a perfect example of the kind of marketing, anecdotes, messaging, and strategies that Mongo employed to stay relevant.

[1] http://blog.mongodb.org/post/49262866911/the-mean-stack-mong...

MongoDB is written in C++, C and Javascript. CouchDB is written in Erlang. It's not weird that it ended up like this.

The language, especially the languages you enumerated, rarely have anything to do with the actual final product's qualities (including it performance, reliability and ease of use). Mongo is unreliable because too much effort was spent making it easy to install and use it immediately (it works!) and little to no care apparently went into anything else. Nothing to do with the choice of C++ for Mongo, or the choice of Erlang for Couch.

I wouldn't be too sure of that. Each language's ecosystem fosters certain ways of approaching and solving problems.

For example, Erlang/OTP brings a lot to the table. Being functional, it would protect you against certain types of errors leaving you free to solve more complex problems. You pay for it with run time performance. String operations are slow etc.

C++ is very fast and very sharp. Not easy to get everything right. (Some sort Greenspun's rule would apply here I think. Substitute Erlang for Common Lisp).

That's pretty much why Couchbase was created if I recall. He wanted to substitute the parts that could be done faster in C and the benchmarks have proved out well if I recall.

I understand it is about tradeoffs, and that the benefits and limitations of every language(in terms of syntax and semantics, and its associated runtime/VM -- if any) definitely define to a large extent the final product's characteristics.

I am only saying that Mongo is not unreliable because C++ was selected; it's unreliable because of architectural choices and lack of care which may or may not have been intentional.

Crudely put, my point was that the people who freely choose C, C++ and Javascript (and PHP etc.) are the kind of people who don't care much for quality. I understand if there are availability or historic (or even sometimes performance) reasons why someone chose a bad language, but if not I'm a bit prejudiced towards them. Erlang, in contrast to the aforementioned languages, is a language for people who plan to do things properly.