I think it might be a generation difference. I grew up with sp games mostly with some games that had co-op and some that added mp. SP was mostly the focus of every game and those people who have grown up with that might not enjoy mp only.
The second thing is price point. We went through a couple of generations where sp and mp were offered in a lot of games. A full package pretty much offered both experiences and now we are moving to more and more games only offering mp. Many feel that by only offering half the content, the value isn't there unless they price it at half the cost.
There is an argument about games like GOW having a 8-10hr campaign, no mp and little replayability compared to something like TF that can give 100+ hours of gaming. In this situation an argument can be made that the sp is so short, like an mp only game it should cost less. However, my argument here would be, and I'll use an analogy here - it's like going to your local pub everyday for only the cost of your drinks, meeting with friends and family or saving up to go to a holiday to Dubai, Hawaii, Serengeti etc. There is nothing wrong with either, one is more cost effective and still offers a getaway with the bonus of having fun with friends, but the other is an experience of a lifetime with deeper culture, history, adventure, locations and overall experience.







