Personally, I don't see much reason to stay level 100. I mean, levels 50 and 100 are virtually identical with the exception of lower numbers, and 50 is closer to the Gamefreak standard. While some may argue about the presence of positive merit of staying within developer intent, it surely has no negative merit, and the average of positive and zero is still positive.
Of course, differences do exist, and I'd argue from my perspective that 50's unique qualities are worse than 100's. The effects of different EVs/IVs are much easier to calculate (presenting a lower skill floor, which is a good thing), and having a Substitute (mostly a tool of setup sweepers, etc) able to resist a Seismic Toss (a tool of stall exclusively) I would have as harder in order to enable more defensive playstyles. In addition, as people have calculated, in a level 50 system, milestones of ohko v 2hko, etc are much more easily hit. Of course, whether these changes are positive is up to popular opinion, and I'd argue the majority of players prefer an offensive metagame.
The question isn't "why", it's "why not".