Comment by CyruzDraxs

It was easier until we discovered even cleaner ways, and most of the Javascript community began to adopt them. But node chose to stick to the "old" ways. (Not really old, but not as shiny and new as the alternatives)

I feel like promises don't need to be part of core though. It's easy enough to wrap the standard library in userland and just have other things depend on the wrappers.