"easily deployable" is part of the equation, but even assuming a clojure app was 'easily deployable', the architecture of JVM apps runs against the interests of shared hosting providers - anyone looking to get more functionality from less hardware.
Keeping JVM engines going, and keeping a lot of compiled apps in RAM, on the offchance that someone makes a request, means fewer apps/sites can coexist on the same hardware. My $5/month PHP plan (hypothetical - I don't have one) - only compiles/executes the PHP when the request is made. If no one requests my site for 3 days, it's all just sitting there, not using any resources.
Until there's another technology that follows a similar model (or something else which provides good economies of scale for hosting providers), PHP will continue to dominate large segments of certain problem spaces.