Yeah - ad revenue is a factor. We have to rely on ads to pay our bills - it's the only way other than subscriptions to make a forum work. With mobile growing as a factor, we can't afford to be ad-free on one platform.
Beyond that - it's a matter of forum experience. We've got a huge amount of customisations over the default vBulletin, I can modify the mobile site to bring those in where they make sense, but with something like Tapatalk, you have to take what you get - and if something goes wrong, I can't fix it.
Custom apps would solve that - but a bunch of members are running on Blackberries or old Symbian phones - we've even had visitors on PS Vita, Firefox OS and Bada. The diversity of our members is matched by the diversity of their devices - a website is the only solution that covers that.