Because there isn't an infrastructure, as there is with AM radio, to have a radiation hardered transmitter and backup power in every geographic area.

Gee, if only we had some sort of other high-availability comms system we could use. Maybe divide the country up into geographic cells and put a transmitter in each one that sends out alerts to portable devices that people carry on them. You could even make the comms two-way so you could call for help if required. I wonder why no-one's ever thought of that?

Incidentally, I have never in my entire life received any kind of alert over AM radio. I have however received several on my phone, generally over incoming severe weather.