There were several reasons, but the biggest one ifs that the vB developers have no interest in keeping up with PHP upgrades and they sit incompatible for ages. They had no plans to upgrade vB 4 past PHP 5.3 until tons of people were complaining about how buggy vB 5 was and they finally relented after over a yer of holding out on it. Now PHP 5.6 is out. Any bets on how long it is before they support that version of PHP?
There are quite a few other reasons like the fact that their new licensing sucks and just forces them to release major versions faster and internet brands is the monsanto of internet companies, but those really aren't the major reasons. xenForo is developed by the old core set of vB developers (so much so that Internet Brands tried to sue them a few years ago) and it mostly has the same set of ideals as vB had back in the good old days so I don't feel bad supporting them. They also have much more reasonable pricing and their licensing is just like old vB but cheaper with 95% of the features we really use.
Sounds good. Yeah, I REALLY don't like Internet Brands these days either for pretty much the same reasons. I think they're just living off the past success of vBulletin 4 now.
Out of curiosity, does it look like XenForo is as feature-complete then as vBulletin 4.2.2? It didn't look like it last time I checked. (which was a while ago)