First, there are several Heisman trophy winners who have gone on to have great, and in some cases Hall of Fame Pro Careers (For example: Doak Walker, Paul Hornung, Roger Stauback, Jim Plunkett, OJ, Tony Dorsett, Marcus Allen, Earl Campbell, Barry Sanders, Tim Brown)
But for many of the other Heisman winners, they were great college players, but that doesn’t mean they will be great in the NFL. First, its luck of the draw of whom you are up against. You may be the best of a particular year, but that doesn’t mean you are an all time great. (take a movie that wins best picture of the year, not because the film was an all time great, but b/c the competition that year was very weak). So you win a Heisman and fans instantly expect you are on your way to a Hall of Fame NFL career...not the case. Second, particularly with QB's in most cases their skills (Danny Wuerffel, Andre Ware, Eric Crouch, come to mind) do not translate to the Pro Game. It was very easy for Wuerffel to just sit back throw bombs and have his WR run under the passes for TDs (The Fun and Gun offense of Steve Spurrier). Doesn’t work in the NFL as the speed of the game is too fast and the abilities of defenses are far too great. Andre Ware had similar issues; he couldn’t make the necessary throws to thrive in the NFL (he didn’t have to make those throws at Houston). Eric Crouch was thought of as too small to play QB, he never really got a legitimate shot. Third, injuries play a part in limiting the success for some Heisman winners (Billy Simms, Rashaan Salaam, BO Jackson for example)and in other cases some winners did not play Pro-Ball (Charlie Ward, Dick Kazmaier)...especially some of the early winners were the money wasn’t that great or they gave up playing to fight for our country.
Bottom line is that more often than not, the Heisman should be viewed as an award for a very good and sometimes great college player, not an indicator of success at the next level