I have no insider info.
If you recall, it was Family Radio when headed by their previous leader who predicted the end of the world and it didn't happen, obviously. Naturally such a claim found to be false is good reason to lose tons of donations from what had been their following. Selling may not have been a choice.
Second, an FM frequency is multiple times the value of an AM signal. Think at least 5x the value, maybe upwards to 10x.
Operating an AM would likely be a bit more expensive but considering all of this and the exclusive content, buying an AM makes sense.