Originally posted by @eladar
Why didn't other mammals develop long necks?
I have in effect already answered this question in my last post here.
Because different animals specialize on feeding on different varieties of food and with different ways of accessing it.
With just a few possible exceptions, few carnivals would get much benefit from longer necks.
Those herbivores that can only easily digest grass but not easily digest the tree foliage in that area generally won't benefit from longer necks.
Those that climb trees (such as herbivorous monkeys) also won't benefit from longer necks as they climb trees and longer necks might actually hinder their climbing.
In areas where low lying vegetation that a herbivore can digest is never in short supply, even if there are also plenty of tall trees in that area, there would generally be little benefit in having a longer neck (unless the tree foliage happens to be significantly more nutritious than the low vegetation) and a very long neck might actually make grazing low lying vegetation harder.
In areas where there are no trees and all vegetation is low lying, there wouldn't be benefit.