The fact that it was one of the highest-selling Devil May Cry games and was also one of the perceived worst. People seemingly bought it on faith alone, and they got burned for it.
It still sold less than DMC1 and there wasn't much resources for people to go by since well Youtube didn't exist nor were websites like IGN as commonly used as it or game reviewing websites and such weren't all that common or used as it is today so it was hard for the public to know what they were getting themselves into. Back most news especially gaming related (ESPECIALLY GAMING RELATED) were spreaded by word of mouth with a majority of the people. If anything DmC's low sales would be a factor of how completely different video gaming media and information gathering is so advanced now so people can get info better.
If this was 2003-2006 DmC may have sold a bit better although people would be confused on what it was (although some people to this day are still confused) and something like DMC2 would've sold worst and DMC3 sold better if released under conditions of now.