Sales has changed. That's all there is to it. This is a good video put out by Huthwaite, the company that brought you Spin Selling.
Ignore the dry and almost deathly boring presence of the narrator, Huthwaite's President and CEO John Golden. It's painful, but if you can get past it, there are some good nuggets in it.
My favorite part come at 3:56 to 4:44 when TAS CEO Donal Daly talks about the death of the relationship sales person.
Another one of my favorite parts is when Dave Stein of ES Research talks about value propostions.
Take the time to check out this video, it's worth it. It does a good job of explaining what has changed while you've been busy selling.