Why is MySQL More Popular than Postgres circa 2010?
Here are a few observations:
For as long as I can remember, MySQL has offered a simple install package for Mac and Windows and a straightforward tar for *nix systems. The Postgres installer package (at least for Mac) still needed manual modifications to perform acceptably. In the private enterprise world the database administrators tend to be different people than the coders. For personal use, many developers probably just prefer something that works out of the box.
Up until version 8.2, Postgres didn't support multi inserts/updates. Basically queries in the form of 'INSERT INTO dummyTable (column 1, column 2) VALUES(1,2), (3,4)...' Which would be a deal breaker for me. You may say it's been five odd years since 8.2 came out certainly we've progressed as a people. For this you need to remember that for many developers shared hosting is the reasonable means to house their web applications, and shared hosting feature-sets are generally years behind the curve of current tech trends. The reason for this is generally not laziness, constant platform upgrades can break running code and there are generally hundreds of web sites on each host that could be affected. Speaking from personal experience, I had to switch the providers for this site since my previous host only supported Posgres 8.1.
No 'Upsert' support. Speaking for myself, this isn't a big issue, though certainly an annoyance.
But time will tell, ultimately...