I think the reasons why people are opposed to yet-another web forum include:

• Yet-another place to look for mail. (This is why some people only show up on PM every few weeks or months ...) We're all very used to checking our in-boxes. Anything arriving there gets our automatic attention. By automatic, I mean that we don't need to do anything.
• Yet-another way to create messages. I'm sorry, but even PM's web interface just simply doesn't hold a candle to editing mail in KMail. And I'm sure most people, when comparing this to their own email programs, would agree. I don't want to learn yet-another interface to typing stuff in. (I also want to find a better way to interact with PM... this text area is annoyingly small...)
• Yet-another way to interact with messages. KMail has collapsing threads where I can see all my threads at once. PM ... does not. Does CPAN::Forum? Probably not. But it does have a different way to interact with messages than PM - so it's still yet-another thing to learn. I've already learned my email program, why learn more?

One of the reasons why mailing lists are so popular is that, once signed up, you really don't need to learn a new interface to get at the new information. Newsgroups? Heck, some software treats them exactly like email, so you get email and news in the same interface - again, no learning curve. And because launching your email client is something that many people just accept as a fact of daily life, anything that integrates into that is already integrated into daily life.