Nashville has numerous viable AM signals. Indy & Atlanta, not so much.
WSM, WLAC are class A stations with good coverage of the entire metro day and night. There's 3 or 4 other AM stations with good coverage in the daytime, and poor coverage at night. (1160, 1200, 1300, which are all currently broadcasting religion, and an NPR talker on 1430).
Here is the problem: the Nashville market is 8 counties. WSM covers all of it with a 5 mV/m signal day and night, but does not cover it all with a 10 mV/m signal... and in Nielsen, indoor listening at home and at work just does not occur outside the 10 mV/m contour. WLAC by day reaches 55% of the market with a 5 mV/m signal, and only about 30% at night... less for the needed 10 mV/m signal.
No other station comes even close to covering the whole market day and night. And that is the problem all across America. The "better" signals were licensed to serve cities before the post-WW II urban sprawl, and most markets have outgrown all or nearly all of their AM signals. And man made noise has reduced the effective coverage considerably, as well.