It's due to the ongoing pussification of our society. Everything must be hyper politically correct for anyone to be allowed to say they enjoy it. We DO enjoy high- and low-brow humor. We're just not allowed to admit it in public, else someone will sue or publicly shame you. For example, if you say, "DNF was mildly enjoyable", a common response from the politically correct crowd might be, "Well, that means you like murdering rape victims. You're a monster and you don't deserve any cake*". Most people decide to keep their mouth shut, and the vocal minority who run all things politically correct win. This is a slight tangent but I think this concept actually makes us less "free" than people in countries where sticks are not surgically implanted in their asses at birth.
On topic, I've played through about half the game. It's a barely average shooter with a few chuckle-worthy moments - all due to "childish" humor. I am enjoying the game as much as I enjoyed playing the last Call of Duty game though, because DNF has more personality and is completely not serious in how it presents itself. I can actually *gasp* mindlessly relax in this game. Call of Duty, et al, are SRS BZNS and it takes more mental energy to play them through.
That said, the game is not worth $60, even the PC version I'm playing. I bought it on Steam for $44US and I think that's overpriced, too.
*Please note that the cake is actually a lie.