I guess it's a matter of business decisions. I know on my site I've used hundreds of dollars of revenue for online software purchases. vBulletin, vBadvanced Gallery, JpGraph and now vB Articles is a commercial product, as well. True, it's more expensive to operate that way. But, it's a sacrifice I'm willing to make for the community.
I actually started with InvisionBoard, which I like better than phpBB. It was free at the time, though I think they now license it out. Ultimately, it didn't offer enough add-ons and it wasn't customizable like I wanted it. Features were lacking.
There are also countless other bulletin board software. I don't know how they all coexist! Survival of the fittest, surely.