Because the developers of the site know PHP and not JSP mostly
This site started purely as an exercise to learn PHP, nothing more. It slowly formed around that premise. It was also a good excuse to learn SQL.
plus613.com runs on a dedicated P4-2.4ghz server with a gig of RAM and 80 gig of storage, running Red Hat Linux with a load average of around 1.50 to 2.50. The HTTP server is Apache. We also have a backup 1.3ghz Celeron server which is not currently in use. Each server has a dedicated 100mbps port on our ISPs switches, the ISP has 6gbps multihomed bandwidth available.