Lazy devs. It's a one-night procedure - add one line of code, recompile.
If you're really annoyed like me and have a chevron unlocked device, you can use Advanced Configuration Tools to force instant resume. Was a life saver when dealing with games like The Harvest, where an accidental press of the search key meant a minute-long wait.
Marketplace should do a better job of highlighting which apps have gone Mango. Personally would rather support devs with commitment.