It's not so suprising. You don't really need the overhead PostgreSQL brings in for most web applications. You probably don't need the overhead of SQL Server, either.
I could be wrong, but I don't think anyone's implying that these sites' success is due to their backends. However, backend choices do affect uptime, scalability, ease of development, ease of finding/fixing bugs, etc. So I would certainly take issue with your claim that "this sort of stuff is irrelevant".