My guess is people are busy with their real lives. SitePoint has always been active, it crosses many programming languages, and is a one-stop portal for anything web design. There are a lot of professional web designers about there as well, thus making it a very thriving business community. vBulletin.org is mostly a portal for add-ons. My guess is most people are content with their vBulletin 3 version without having to install hacks for it, thus aren't spending the time at vBulletin.org as they had with vBulletin 2.
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