Hosting gets mentioned lot and it was true at one point. Honestly though, that statement is no longer true as far as I can tell. It may be that there are hosts that only support PHP but it is certainly easy to find good quality hosts at any price point that support lots of other stuff as well. Ruby, Python, Perl, ASP.NET and others are all pretty common even on $7/month plans.
If you pick a host at random, I expect that they will support ASP.NET at the same price points as PHP.
"Finding someone who knows it" is an interesting one. Skills in other languages are not exactly rare but I imagine that it is probably true that there are more people exposed to PHP. If we change the task to finding somebody "who knows what they are doing", it could be argued that it is no easier to find this in PHP-land than elsewhere. In fact, the task may even be easier for other languages.
Deployment is also interesting; I could say a lot about it but I will give you that one. Well, I will say that if you find a dev "that knows what they are doing" it pretty much becomes a non-issue. One of the things that many people (that I have known) really love about PHP is the ability to edit in-place in production. That is of course really nice and completely horrifying at the same time.
PHP obviously fills a need as it keeps on going strong. One might expect a competitor to emerge that addresses PHPs warts while offering it's attractive characteristics. I think that your three issues ("broad exposure, ubiquitous hosting, and no-fuss deployment) are big factors. If I create a "better PHP than PHP" I will not attract many Node.js, Rails, or ASP.NET developers. Sadly, the lack of millions of tutorials and hosts for my "better" platform will keep the would-be PHP devs away too.
PHP keeps evolving though. With the addition of traits it has something many languages lack (including C#). The need to stay compatible holds the language back but someday (far from now) there may be a subset of PHP which is nice. Keep your eyes peeled for the book "PHP: The Good Parts".