If you think about how these providers deploy a cloud facility, it makes sense that the offerings in a given place are relatively static. The whole network design, thermal/mechanical design, and floor plan is built with certain assumptions and they can't just go in and rack up some new machines. It evolves pretty slowly and when a facility gets a new machine it is because they refresh the whole thing, or a large subset of it.
That said, the EPYC machine type is available in 12 zones of four different regions in the US, which isn't bad.
