You're missing the point.
Djokovic made a huge improvement in results in 2007 compared to the previous years so he deserved all the hype. Then went on to win the 2008 Australian Open which wasn't a surprise at all. We all knew it would come sooner or later. Obviously, has been a top 4 favorite for just about (if not every) major since and now he's as close to a lock for a slam tournament since peak Federer.
Del Potro didn't even get the chance to be the favorite for the 2009 US Open, maybe 5th or 6th given his ranking and a final in Canada Masters a month prior. His win over Federer in the US Open final was a BIG surprise.
Murray has been in the top 4 since 2008, he deserved to be one of the favorites for maybe 1,5 - 2 years. Not now. Not when he failed every single time, often mentally which means he doesn't have the right mindset to win majors. And in most cases you can't improve that. As I said, it either comes quickly, or you're gonna need a miracle later on like Ivanisevic to actually win one.