It's a number of things:
Negative comments from 01 FFL's who don't like that 03 C&R guys can buy them off the Internet or mail-order cheaply. These attitudes get picked up by other customers not interested in milsurp.
Milsurp is an affront to people who try and buy their way to marksmanship, or have a significant portion of their ego wrapped up in their new commercial purchases.
Simple ignorance that they're "old" and "obsolete" and that "new" is automatically better.
Some of the angst against milsurp also comes from the older generations who grew up with milsurps that sold for a few dollars out of barrels at the local Woolworth's. For them, milsurp was seen as a sign of poverty, or cheap beater guns for kids, and the trunks of cars where rust or theft was a concern. They also represented military service, not so much trauma/PTSD, but where you had no choice in what you were issued, and commercial firearm choices represented "freedom" to be your own man...
And it's as others have said, you can find the same attitudes about cars, computers/game systems, motorcycles, etc. It mostly just boils down people with a weak ego that needs reinforcement by criticizing something else.
These topics are always interesting to me, as I've never really been the direct recipient of someone bashing any of my particular firearms, and I have a wide gamut of milsurp, antiques, modern, inexpensive, and expensive items in my collection. Either I'm lucky, or I just don't get out to the range much. When I see someone shooting one of the items I consider "inferior", if the item actually gets discussed, I still try to say something positive, unless it's clear the owner truly wants honest feedback.
The only real firearm type/make bias I've encountered is a co-worker who's convinced the AR/M4/M16 platform/family is crap, and will state so vehemently to anyone who asks him. However I cut him some slack, as he was serving in Vietnam right at the time they got their M14's taken away, and the M16 was introduced with no cleaning kits, and 5.56 loaded with the incorrect ball powder for the gas system. And he rotated out before the problems were rectified with proper rod powder in the ammunition, cleaning kits, and the improvements in the M16A1.
I cut him some slack.