A lot of people on this site don't PM out of preference but necessity.
The Guild get's ad funding/support for the site which keeps it running, but if the people paying for that find too much smut/mature content on the site then that funding goes, and the site goes.

So people who do such RP's are required to do it by PM or they risk getting banned.

As for me personally? I stopped taking part in those kinds of RP's since I was about 16, so I have no reason what-so-ever to go to PM for RPs. 
I'd much rather do the forums, simply because it's easier to keep track of.