The advantage it has is that you can run a ton of hosted sites on one mod_php apache server. With Rails, for example, you have to run an environment. Same with many Python apps, or Java, etc.

So it has the memory advantage of being able to run multiple sites on one environment.

Not that big a deal if you're doing any serious deployment, but a big deal for its ubiquitous nature.

90% of people just run a web site on a hosting server. Granted it's getting less and less common with the advent of the centralized service-based web, but still, that's the source of success.