Outside of the startup community developers don't have such a great appetite for risk, especially with their data.
I'm one of those people - I just know My SQL better. It has nothing to do with ease of installation or building queries. It has to do with troubleshooting. I know how to tune MySQL and troubleshoot. I know how to install and use other systems but it introduces risk for me to put them into production without a very compelling reason.
I would happily use another system if there were a db admin to take responsibility for running the server. In fact my favorite db is Oracle, but I have no experience as DB admin on that platform.
I was involved with one project with PostgresSQL that was successful around 2005. I did 5 "shootouts" of DBMS alternatives that considered PostgresSQL and other products for various projects and despite having some attractive features, PostgresSQL always lost out. For instance, in mysql I was able to create indexes that could handle Freebase text fields without any "impeadance mismatch and it wasn't clear how to do it with pgsql.
Fact is that simplicity is more important than features for most people.
Configuration is a different story. When you are done with a Postgres config you have a more secure setup. They don't allow you to shoot yourself in the foot by default.
Honestly, installation as a differentiator between databases is a seriously weak argument. If you're going to troll pick something a little more technical like acid compliance or something else that nobody seems to care about.
And you ignored Windows, Mac as well as the huge number of LAMP based installers. Being able to quickly get the entire stack up and running very much helped the popularity of MySQL.
Meant no offense. Seemed trollish but when you factor in development setup then yes MySQL is slightly easier. I think heroku is doing a good job making Postgres a snap.