I think there are lots of reasons people buy large magnums for (mostly or entirely) deer hunting. Marketing for sure is a factor. Bigger is better mentality. There are those that want the one gun for everything and buy the magnum "just in case" they ever go out west for elk. There also seems to be an attitude among some that lighter deer calibers are for children and women.
I know three guys who hunt deer here in PA (one in WV also) with a 338 win mag, and another who uses a 300RUM (also hunts in upstate NY). When I have inquired about their cartridge choice, they have said "I may go out west some day for elk" or "I sometimes hunt over a large field or powerline right of way and need the extra power for long shots", or "I don't like trailing deer and this puts them down right away". With that said, however, most people I know use the 30-30, 243, 270, 30-06 and a few the 7mm mag. My buddy shot a deer here in PA at less than 100 yards with his 7mm mag a few years back, and it massive tissue destruction and a volleyball sized crater on the off side. Bullet was a Corelokt, I believe - he doesn't buy expensive "shells" like a ballistic tip. He has gone back to using the .243 for deer now and uses the 7mag when we go black bear hunting.
Personally, I've been hunting mostly with 30-30 and 7mm-08. This season, I'll be taking out my new Kimber 84M in .257 Roberts. Only on nice days though!