First, from the UBC standpoint, while there were potential advantages to joining the NCAA, there were also significant issues that hadn't necessarily been addressed. They could have potentially linked up with the Division II Great Northwest Athletic Conference, which has become a home for fellow Vancouver-area school Simon Fraser University (located out in Burnaby), but GNAC (and all of Division II) notably doesn't offer hockey. UBC could have tried to play up in Division I in hockey, but that wouldn't have been all that easy; gaining admission to a Division I hockey conference isn't the simplest thing in the world, especially for a school that doesn't have an NCAA track record. If the school had been able to guarantee that they'd be able to play Division I in a few sports (particularly hockey, but perhaps also volleyball), that could have been a factor in favour of moving, but from what's come out so far, Division I always seemed like more of a possibility down the road than a lock.
If Division I wasn't a real possibility, there wasn't as much in favour of this move for UBC. SFU received some media coverage this year from the Division II move, but not noticeably more than they'd received while still in CIS, and some of that was certainly thanks to the novelty value. It would be tough to argue that playing Division II in all sports would be a huge step forward for the university, as Division II sports don't generally draw a lot of attention (and UBC would have to start building many of their programs over from scratch thanks to the different eligibility guidelines of the NCAA). Moreover, there was plenty of opposition to the move both on campus and locally, so a move wouldn't have necessarily been all that popular. Moving also would have taken away one of their recruiting advantages; with SFU now in the NCAA, UBC had a wide and fertile recruiting ground to draw from for those who wanted to play CIS sports.
By staying, though, UBC seems likely to actually fulfill many of the goals that led them to consider moving in the first place. Since UBC and SFU first threatened to leave, CIS has substantially evolved, and it's generally done so towards the bigger, more professional vision they articulated; CIS today features more athletic scholarships (especially in Ontario), better facilities, more experienced coaches and generally a more professional environment than it did in 2008. As Justin pointed out, progress hasn't been the fastest, but there are suggestions in UBC's press release that further movement towards their goals is coming. Here's what president Stephen J. Toope had to say on that subject:
UBC’s consultation process has, I believe, contributed greatly to preparing the ground for the changes that are required for CIS to become the effective competitive arena that all parties want it to be. CIS leadership has declared a goal to be the location of choice for Canada’s best student athletes. To fulfill that commitment, change is required. During our consultation process, and as a result of conversations across the country, I believe that change is possible.There's no word yet on exactly what those specific changes are likely to be, but they're believed to likely include further expansion of athletic scholarships and possibly tiering. UBC and other large schools are also likely to be given more influence in overall CIS decisions. Each of those potential changes carries its own issues, but they do all perhaps offer benefits for the whole organization. What indisputably does offer benefits for CIS is UBC's continued membership, though. The Thunderbirds have long had some of the most distinguished and recognizable CIS programs, and the news that they'll continue to play north of the border is a boost for the reputation of the entire league.