The forum was originally on Zetaboards, and we found a functional script that could convert it to SMF without too much hassle.
Since then, it's been a case of "it ain't broke, why fix it?" I think - the time, effort, and loss of people/activity that is associated with moving an entire forum is pretty large, and making a change that big never seems worth the community disruption. I don't think we have many complaints about SMF either, the bulk email system is terrible but that's the only thing I can think of that tends to actually a be a pain.
