i think a lot of the reasons I've seen come down to business reasons, not technical. Someone wants an app fast, like now, and MongoDB is fast to setup and get running with.
yes, it must be the government's fault that the people don't have jobs. it couldn't be that they have priced themselves out of the market with the prevailing wage in Spain vs. China. and, of course, they couldn't possibly start a company to employ themselves. it must be the government's fault. let's protest.