It's because most people on the internet who like to speak their minds through comments and discussion are usually Battlefield 3 fans/fanboys. This is why on most videos involving MW3 or BF3 almost always has arguments between the two, with BF3 almost always being more liked among the commenters.
Most people like to make the generalization that MW3 has "just a few things changed." They usually have no evidence (besides, of course, "only a few perks and weapon changes ololol!") to back up their claim and most just say it because their going with the bandwagon.
It's all about the Battlefield series being the "underdog," and people like the underdog. With the release of COD4 in 2007, the COD franchise got insanely popular for some reason.
When something becomes way too popular, has little kids & douchebags playing it, has pricy DLC with content that doesn't live up to the people's standards, it forms into something people love to hate.
There's also the talk of Battlefield being the more realistic war game for the use of vehicles such as jets, helicopters, tank, etc.
In the end, Battlefield then has a cult behind it, and they make sure to point out every flaw with MW3/Activision/Infinity Ward to make them feel like they are superior because they feel they are playing a smarter, more realistic, underdog of a game.
Those are my two cents.
By the way, I respect both of the games. They both look great. I prefer the COD franchise though. I could never really get into the Battlefield series.
