Will Fly: part of the problem - maybe the major part - is that the deers' natural predators (wolves and the like) have been pretty much eliminated except in restricted areas because they also attack livestock. Another factor is that farmland provides an area for them to feed that they didn't have when the land was in its "natural state". The result is basically unrestricted breeding.

Another problem is that many people see deer as "cute", as a result of Disney and others of his ilk. I used to live in Erie, Pennsylvania, and there is a nice little park there called Presque Isle. It used to be an island, but they put in a short causeway and a road many years ago. The deer are essentially trapped in the park, where hunting is forbidden (the surrounding area is very built-up), and it is necessary to seriously cull them every few years. The city folks get really upset at this!! I guess they think Bambi is better of starving than being shot.