Back in the late '80s/early '90s, it wasn't unusual at all to find AM rock/metal and alternative stations. Some of them enjoyed small, but rabidly devoted audiences.
Then some FM stations began picking up these rock formats. And well, once these were established, the AM rockers died off.
The Z-Rock network was made up primarily of AM stations, usually second tier stations that failed at just about everything else. And it worked for a while...or at least until an FM competitor came along. Or new owners. The Z-Rock network itself (which was going through some massive changes due to the demise of hair bands and the rise of alternative in the early '90s) finally announced in October 1993 that it was dropping all AM affiliates, which was roughly 2/3rds it's total number of affiliates. The Z-Rock network was dying, but this move seemed to accelerate it's demise more than the changing musical tastes.
There are still a few AMs that break the typical AM radio mold and play rock (or upbeat AC.) But they're usually in smaller towns where there is little to no local FM competition.
Most of today's thrash metal is pretty lo-fi and would be right at home with AM's limited fidelity range. But it's a tough sell. If there's a visible hard core of rockers in an area, it has potential. But most radio business people are yuppie suits who don't want to be associated with "undesirables" (as I actually heard one of these suits call hard rock fans. Tsktsktsktsk!)