There are several reasons for them not being found and convicted. I think one of the biggest ones, pirateat40, got prosecuted. They also managed to get Ross, so those who are treated as "high profile" are found.
As for this forum, I believe that at least half of the accusations don't even mention money. The people involved are accused of hacking accounts, scamming signature campaigns for a couple bucks, taking loans and disappearing... That 10% (at most) that steal big money, still can be considered small fish. Nobody is going to spend days or months trying to trace wallets when there's only a couple thousand USD at stake, but BTC is by all means traceable. You just have to spend the coins somewhere for it to work. If the thief hoards the coins tracing is pointless.