The way I look at it is like CUM says - if NFL coaches believe it can help them win without getting their QBs killed, they'll use it. So far, the Wildcat has been the only variation of it implemented and only then because a RB is lined up as QB.
The Wildcat isn't really a spread offense. There are 6 men on the line, 2 backs, and usually 3 ends with one of them lining up too close to the offensive linemen. The defense isn't being "spread out". It helps offenses run the football better, but there's no real threat to pass.