The question is actually a fair question. I think what is being asked is outside of the UX what makes XF a more powerful forum software. This question be suited for someone that has developed in both phpBB and XF. I don't think XF has had the success it has just because people like the layout of the forums or admin panel better. It wouldn't be too difficult to make any forum software look like XF.
I have worked on custom themes for both, and XF is definitely more user friendly than phpBB. The underlying code is better, the software has more features, the software just feels more modern, and overall I recommend XF if you are able to spend money on your project. If not, phpBB is a good choice.