Because, apparently, vB 5 is a 'built from scratch', meaning that instead of evolving vB 4, they trashed it and started over, like they did with vB 4 after vB 3.
There are some legitimate reasons you might do that, in terms of code cleanliness perhaps, but otherwise it's generally an admission that what you had before is so bad it can't be fixed.
In this case, they started from fresh, didn't have the memberlist code and decided they weren't going to write it. Notice the lack of 'it will be added in future versions', almost conspicuous in its absence, unlike the warnings system which is merely a 'shit, we ran out of time' moment because they rushed it in an attempt to get it out the door prior to the lawsuit wrapping up. Unfortunately it may have cost them more dearly than the meagre (by all accounts) sales they did manage to get out of that exercise.
Mind you, phpBB 4 is in a similar situation to that, they're not going to upgrade phpBB 3.x, but phpBB 4 is a 'from scratch', not that phpBB doesn't need a serious overhaul or anything ;) Though it's a bit different when you're talking about a group of volunteers making something vs a company pushing out a paid product.
:edit: vB 4 was not 'from scratch' entirely because vB 3 was a trainwreck that couldn't be fixed, it was that the code was perceived to be creaking from being too powerful and too tightly woven and they wanted to uncouple things somewhat. That, in itself, is fair enough reason to do so, only it needed to have been done properly, something which Internet Brands wasn't willing to pay for. XenForo, effectively, is what vB 4 would have been had Kier and Mike been allowed to build it their way rather than outsourcing coders and letting head office 'quality check' (which is what happened with vB 4 and that was a buggy POS when it landed, but nothing on the scale of vB 5)