I think it has a lot more to do with the way AMD CPUs work.
It's been proven that there are performance gains by running your infinity fabric clock up to 3600mhz, because it benefits the CPU as well as the RAM. Not so much the RAM, but more the CPU itself.
So I would imagine going from 3600mhz IF to 5000 and beyond would have hindered the CPU performance. That said, given it doesn't use a traditional memory controller it may not even be possible to make it work on DDR4 any way.