My two cents: because bridging two independent software products is usually a bad idea. You have 3 pieces of software to deal with: Software A, software B and the third-party bridge. It might be called “seamless”, but it never really is. Anytime you install an update to software A or B things can break and you need to hope that the third-party offering the bridge can fix it in time. And since they are usually small companies, they might simply stop offering the bridge altogether in the future and then you are completely screwed.
Personally, I use either WordPress or the Invision Community Suite, never both. If I had a WordPress installation already, I would convert it all to Pages. After all, that’s the benefit of having a Community Suite. It being seamless is not just marketing claim in this case.