Because RP fights can be very lengthy, and can often end up in OOC conflict. This is especially the case when one party wishes for PvP whilst the other wants RP, because they are generally much more bitter towards each other and, like before, leads to OOC conflict about whether or not an emote is PG etc. PvP default ensures that there's minimal OOC conflict, at least during the fight itself, and there will be no powergaming to cause it.
Also, many people often say "I came to an RP server for RP". If this were the case, there would be no such thing as OOC chat. I personally came to the server to have fun, and OOC conflict and the possibility of powergaming, as well as extremely lengthy battles, even if they do go smoothly, is not fun for me. Also, this doesn't stop you from RPing. It just means that if someone wants to PvP and you don't, they get their way over yours. This will not be the case in a lot situations, because as many have said, a lot of people still prefer to RP.