Frequency/amplitude modulation and SNR
Signal to noise goes as you'd expect, but in FM it's rather more subtle.

Let's say a transmission has white noise added to it in transit. That noise is uncorrelated to the signal, so half of it looks like AM (in phase with the carrier) and half of it looks like PM (phase modulation).

If the want to improve the SNR due to AM, then we can increase the carrier power of our transmitter. This gives us a linear improvement in SNR at the receiver, at the cost of real transmitted power. This is measured in watts, and so money for bigger transmitters, more power for them etc. If the interference is from natural sources, then this works. If the interference is from other transmitters further way, then 'everybody shouting louder' does not work.

If we want to improve SNR due to PM, then we can increase the carrier deviation of our transmitter. It costs no more in watts to send a signal that has +/- 10 radians deviation than one that has +/- 1 radian deviation, however the first one will have 20dB better SNR. You don't get owt for nowt, and the cost of increasing the deviation power of the transmitter is that you can cram fewer wide deviation transmitters into any given bandwidth than AM transmitters.