Haas has more talent Davydenko - he and Nalbandian are pretty similar talent wise.
Haas has had a ridiculous injury run at horrible times in his career and just general poor luck. In 2002 he was in glorious form and had two big chances to bag a slam. The AO was just a fiasco with the rain delay against Safin. He was one of the favourites going into the USO but was stiff to run into Sampras and lose a very tight 4 setter. By the end of the year he can sustained a serious shoulder injury which which see him out until 2004. Shoulder injuries are arguably the worst injuries you can get as they seriously damage your ability to serve. By the time he recovered he pretty much lost his window with Federer arriving and winning everything under the sun. He has never been the same player since and has had to adjust his whole game because of the injuries.
Nalbandian has less excuses...he should really have a slam right now but squandered some key opportunities. He was never winning the final against Hewitt at Wimbledon though - he wasn't ready. Robbed at the USO against Roddick and in tennis you don't get too many golden opportunities. His career is littered with puzzling losses which can only be put down to some mental dysfunction. Without Federer around he probably would have won a slam or two as well.
Davydenko ...pretty sure no one has expected this guy to win a slam. He is definitely one of those SF type of players.
Great summary of Tommy's Career--injury after injury--not just one, but three surgeries on his right shoulder
Most recently a hip replacement to be added to the list. Actually he has had so many injuries plus illnesses in his career that I have lost track!