Why does decreasing ion channel density result in an action potential not firing?
It is very straightforward. More the density of sodium channels, more the influx of sodium ions. More the influx of sodium ions, more the depolarization. If there are sufficient sodium channels, then the large influx of sodium ions will cause membrane depolarization above the threshold value, thus resulting in firing. If the density of channel is reduced, the depolarization value will not cross the threshold. Thus causing a net change in membrane potential, but not firing.

Potassium channel acts in coordination with sodium channel. Only if there is an influx of positive sodium ions into the cell, potassium channel will act and release potassium ions outside the cell in order to maintain the membrane potential. If sodium channel is reduced, no further processes happen, thus no firing.