Split keyboards have been around for quite a while. You can even see the Kinesis Advantage keyboard featured in films like Flubber and Men in Black and plenty of companies have made ergonomic keyboards.

I think there's a sort of hobbyist renaissance going on right now, though. Plenty of new keyboards are being designed and built because it's more accessible than ever to design your own pcb or if not that to get someone else's design printed, which along with the general mechanical keyboard enthusiast market, has given people a wealth of options to choose from.