For one I think it is a matter of convention and what people feel comfortable with because they are used to it.
But one big problem I see with present tense is that it makes it difficult to switch between scenes, as everything would be written as it happens right "now". Lets say you write about some guy going to work. You can easily tell that in present tense (he walks, he steps into the subway train, etc.). But suppose you also want to switch to his kids going to school at the same time. If you used present tense for that too it might run the risk of implying that both scenes happen at the exact same time, in the very same moment. Sooner or later you are bound to put things in context with past-tense constructs like "after her dad had left for work..." and so on, but getting back to present tense from there would make for a rather uneven read.
I think it's the same when it comes to roleplaying, only that it has the added problem that present tense can come close to godmodding - or at least give that appearance. The moment you say "X does Y" you set it in stone. It happens. There is nothing that can be done about it. We see it happen, therefore it is what it is, no way around it. Of course things written in the past tense still stand as they are written, but I would say they don't have the same "in your face" quality present tense conveys. You can not interrupt what is happening right now until the post is finished and that makes everything written a fact, without any way to get back and change it. Past tense feels more fluid, less set-in-stone, which might make it preferable for roleplaying where several people interact in a format that has to be chronological by nature but where past events can still be influenced by actions that are posted later.