I wanted to title this article PostgreSQL sucks, but that wouldn't have been fair to PostgreSQL has it really doesn't suck, exactly. It is a very well featured database, and was the first open source database to have many advanced features. That said, MySQL from the start has been easier to administer, and generally easier to use as well.
Before I dive in further, I should warn that while I've used both MySQL and PostgreSQL for years and have extensive experience with both, I have dealt with MySQL considerably more. This is due mostly to the fact that most projects I work on tend to use MySQL as their database over PostgreSQL.
After years of using them both, I have had much fewer problems out of MySQL. Backups and restores, even the complex binary ones done with LVM have gone mostly without issues. I can't say the same with PostgreSQL. I've had MySQL problems for sure, but I seem to run into PostgreSQL problems with far more frequency over things that really shouldn't have been problems in the first place.
One could make the argument that since I spend more time with MySQL, I know it better and thus know how to avoid more problems than I do with PostgreSQL. I'll buy into that as well, at least in part. But I see far to many developers pulling their hair out on public forums and newsgroups over how to fix a problem in PostgreSQL that simply doesn't seem to be a problem on MySQL. That isn't to say that people don't have problems with MySQL as well, they do. But considering that more people use MySQL, you would expect more problems, but you don't really see that. In fact, it's the reverse.
Could a simpler path to administration and programming be why more people use MySQL? Probably so. Back when it lacked features that PostgreSQL had, it still was more popular. Its ease of use had to have been at least a significant factor in many decisions to use it.