Cause SMF sucks, basically. So many useful things we can't do with it, like ferinstance just order the damned forums in the index by last post time. That'd be cool, eh? Also, if done right, we might actually recover all or much of Deb's link collections among other things.
More than that, though, it's just a more used and so better supported project. I was a damned fool for pushing Mike into it for dumb reasons.