Proboards a while back decided we could no longer sell airsoft guns in the commerce section which was a huge PITA for everyone involved in the community. And since Proboards has their own terms it really wasn't a long leap in logic to assume at a later date they might not want us even discussing replicas on the site.
Years ago there was talk about moving to our own server but at the time Proboards really wasn't giving us any rules we couldn't live with and hadn't caused any major problems. But over the years they have gotten a little worse each time. Downside was they wouldn't let us copy any of our data, so we had to start fresh with the new site.
The good news is now we own the database ourselves, and going forward we're able to change things, move the data move the site etc without any issues. So if 2 years from now we decide hey Simple Machines isn't the best platform for us to use on the backend we just convert the database to a new forum software, install it and go on with our day. With Proboards you're stuck with whatever they want, and if they get pissed and say delete a forum in it's entirety we have zero recourse. When they decided people could no longer talk about weapons a while back, knives in this case. They removed several fairly large knife forums overnight without any warning.
So lots of reasons but I think despite the headaches getting everything back in order we're much further ahead running the site ourselves without someone looking over our shoulder.
Take care!
Luke