The question was regarding why the deer's neck fits the lion's mouth. It seems unlikely that the fit is to the deer's advantage.
Sometimes you do see an "arms race" situation in nature. Classic example: cheetahs are very good at running down gazelles, but gazelles are very good at running away from cheetahs. Both are evolved for speed.
Some kind of match/mismatch might have erupted between the cougar's mouth and the deer's neck, but it's a sideshow. The main strategy for the deer is of course to keep his neck out of the cougar's mouth in the first place. There are probably other pressures keeping the deer's neck about what it is.
The question was why it wouldn't evolve to NOT fit. I think it's a valid question, but it does have an explanation. The answer lies in the population ratio of prey to predator. A deer's adaptive advantages are more efficiently geared differently from the predator's, whose survival of species is more tied to its capture of prey than its reproductive or other functions, unlike the deer.