Forums are A LOT of work to run trying to make sure everyone is playing fairly. I've been on another one for many many years (for something else) and I've seen them have to clean out quite a few times due to people running a muck.
My husband's a mod on one of his forums. It's a pain trying to keep up with all the posts and who's doing what they shouldn't. He's had people calling in the middle of holiday meals with high school drama type stuff. A few bad apples really spoil the fun quickly. I totally get not wanting to keep up with all of that.
However, if it is really because someone else started a new forum, that would be very hypocritical (it's OK to have multiple partners, but not multiple forums).