Well, there could be something said about eating one's own dogfood, so they could go with phpbb.
At any rate, I suspect that the big problems are:
1. It's actually acceptable, so why pay the bucks to change it.
2. At this point, it's feature-poor enough to be relatively exploit-free.
3. They probably wouldn't be able to migrate content.
4. All the old-time posters might be reset to zero.
On the plus side, they'd be able to implement better search - or maybe just embed google search. And maybe implement a karma system.
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