Its pretty clear what's going on: AMD doesn't have the resources to support all the GPUs they release on ROCm. They focus on the "MI" line of cards, which have similarities to the consumer GPUs. (Ex: Rx Fury is very similar to MI8).

Because MI8 and Rx Fury have similarities, AMD gives ROCm support to both. Ditto with MI25 and Vega, and MI6 and Rx 580.

EDIT: With "Navi" gaining features in ROCm repos, my bet is that a new MI-card based on RDNA2 might be coming out soon. Maybe its Arcturus (the Exascale GPU for the Frontier supercomputer).