I think because lack of developers. I don't how much developer in xperia p, sola, u, and the other xperia nxt. but it's less than the other dedicated forum.
my previous device is xperia x8, it has many beloved developers (ROM,tweaks,kernel,etc), this why it deserves to get dedicated forum, maybe this forum should not be xperia S, but xperia NXT, so it will be more general forum than xperia S forum.
just remember this is xda-developer.com not xda-device.com.