Forums are PHP-based because everyone supports PHP deployment. Most people who are going to run forum software don't have a lot of money for sophisticated setups, or even a simple VPS, so you're generally stuck with whatever your shared host provides. Very few shared hosts provide support for anything besides PHP.<p>PHP, for all of its troubles, is still the easiest, cheapest, and fastest to deploy, and for that reason will keep a strong foothold until something matches or beats it in that area.