Postgres is the C++ of databases. It is well thought out and designed. It seems overly complex at first until you actually use it a lot to solve hard problems, then you understand why it is the way it is and just how great all those features are.
Considering I think C++ has many problems because it is overly complex because it grew on top of a very low level language, I have to say I can't agree with your statement. Just because something has features, doesn't mean it's great, because some of these features can make everything unstable. And PostgreSQL is great because it is solid and reliable and doesn't have anything like that.
Not quite sure what RDBMSs you are comparing Postgres to here... Firebird? Microsoft SQL/Sybase? Oracle? Because none of those strike me as particularly "simple" products?
Almost certainly MySQL, which is overwhelmingly the competitor in the psql space. A big reason why mysql took off is the same reason that PHP took off -- it is very quick and easy to take the first steps.
I think a large part of that simplicity can be attributed to the lack of Windows port of PostgreSQL at the time that PHP was gaining traction. MySQL was the only reasonable option then. And then the community mindset was solidified.
I think it was the need for manual vacuuming. If you were running a small web host at the time picking MySQL over Postgres meant a huge reduction in potential support tickets related to non-vacuumed databases causing customers to run out of space.
That certainly didn't help, but if your DB was large enough where vacuuming mattered, you probably found a cron solution. The other place I saw MySQL win was in single access benchmarks that were scattered across the Web. Postgres would win in concurrent scenarios pretty handily, but people were glued to those serial benchmarks.
