the olympics just doesn't hold the same cachet for top players as the Grand Slams, looking at the lists of past tournaments. Fernando Gonzalez twice got medals (2004 and 2008), but only reached one quarter final in the Grand Slams in those years...suggests the others weren't trying as hard.
There's some truth in that, but the Olympics is predominant a 3 set tournament - which will favour certain players. You could actually look at it partially the other way, maybe Gonzalez was looking at the Olympics as something he could realistically win and so looked to peak there in those years. Him reaching the final of the least prestigious Grand Slam in 2007 also backs up this idea I think.