Supply and demand and perception is pretty much what it boils down too. There's a lot of PHP apps, and a lot of terribly written PHP apps. Add that companies that hire for such positions have a plethora of options for cost-effective (read dirt cheap) and a lot of developers that will work for such wages and you have a language that while popular, is not well respected, and has those looking for more money and a more versatile skill set seeking greener pastures.