It is much documented the many QB's drafted in the 1st round have been busts. This is somewhat overstated and many other position drafted high been busts, but is many top picked QB's turned out to not be back-ups in the NFL.
There are several causes of QB's bombing worse than other positions.
The higher expectations put on QB's: The QB is given too much credit for victory and too much blame for defeat. QB's are seen as leaders of the team so when the team fails the owners, fans, media and coaches blame him. Many times when heads have to roll his is first to go.
The change of pace: Colleges run simpler game plans with weaker athletes, some good teams will feature one NFL caliber defender some have two, great teams have three. Usually top college QBs have a set of very good players around him and top QB's on a decent program can simply avoid throwing at the good defenders and target his other stars. Many colleges run schemes that are effective, inflate stats but are useless in the NFL. Every NFL defense has a defensive wizard who will try anything and everything to destroy a young rookies QB's mind and is manned by top athletes at every position. Go ahead and try the run-and-shoot against Rex Ryans Gang Green Jets. Ask poor Matthew Stafford who was cruelly thrown to Dom Caper's wolves on Thanksgiving.
The NFL offenses: The NFL offenses are very complicated and many QB struggle to learn them and any mistake they make on the field are obvious and many time will result in points for the other team and losses for you. If a RB misses a block maybe the QB can scramble away but you can't hide a throw to the wrong jersey. The other part is leadership, making mistakes and a lack of understanding while trying to lead can easily break the confidence of teammates, lose coaches jobs and antagonize fans. The best example of that is Eli Manning who before the Superbowl run lost the confidence of his teammates with repeated mistakes.
The Teams: QBs are dependant on their teammate so if guys drop balls or gives up sacks at inopportune times it can really hurt a QB. The best teams usually have a QB so its rare to go to a good team how Aaron Rogers did. They often end up on the worst team which are usually the worst for a reason. Then they struggle because the team is struggling. Don't look at Flacco, Ryan or Roethlisberger all those team had dominate running games and defenses that just needed the QB not to lose. Many top draft pick QBs are thrown into the fire too early as fans and owners demand we are not paying all this money for him to sit on the bench. Example David Carr who was beaten to a pulp as the Texans QB and never recovered.
QB ego's: QB's as stars since high school or peewee league many of them have never really known a crushing defeat and impossible odds. When they have to sit, study, listen and maybe wait on the bench and their puffed ego can't take it. Some can but a lot can't. The other problem is if a QB gets ego problems as leader he can take the whole team down much more easily. He is dependent on the others if he stops listening to the coach many time the other players won't listen to the coach either and the team breaks down. If he won't practice as a young QB he won't have the information he needs to win. If the player won't play around him he can't do anything. He'll get sacked if the line doesn't block, he'll have no passing yards if the WRs don't run the routes. For years Jim Kelly's ego held the team in check despite their talent they massively underachieved and were known as the Bickering Bills. An ego maniac RB or receiver can continue to contribute as long as he shows up game day. Look at WR T.O. or RB Clinton Portis giant ego's but they can still help the team win, at many of the other position while character is a benefit its not a necessity.
The pressure: All of these thing about the game itself not to mention the new found cash can create unforeseen problems to these young men. Sometimes they can't handle it all. Kerry Collins inability to handle the pressure famously destroyed the Panthers. Tennessee QB Vince Young supposedly had a serious depression and an incident with a gun after being benched.
An example of being the pressure they are under. Ravens Raider late in the 2009 season Jamarcus Russell who had already been benched for poor play now demoted to third string was forced into the game because of injury. The crowd showered him with boo's as he walked onto the field. Now he was facing one of the leagues toughest veteran defenses fighting for their playoff lives with one of the worst offenses famous for dropping passes, letting the QB take a beating and general ineptness. He started off well and marched the team down the field. Then as he was leading the team, on what looked like a scoring drive that would give the Raiders the lead, a Ravens blitz got through. He tried to get rid of the ball, was hit as he threw and the pass went off target and was intercepted. The Raiders lost and the crowd booed him mercilessly.
The biggest reason is at QB its all or nothing. You are a star or a bum. A 1st round pick QB who doesn't do well generally can't find away to make himself useful until he get another shot. A running back who isn't quite NFL ready can be a punt returner or a third-down-back, a corner can convert to a safety. You see guy switch from linebacker to defensive end. Many times talented guys play there way back in to the lineup but its much harder for a QB's to get a second chance. Multi-talented Pittsburgh QB Kordell Slash Stewart was an exception to that rule.
Other positions have no stigma for being benched, a running back who is benched will still see action later in the game or in the year but you don't bench a QB for a few plays. Teams rotate linemen, bring in extra WR, a let running back out for a breather but not QB's. WR Crabtree came-back to start after his ill-advised money driven ego trip. A rookie QB would not be able to recover to start that season and his career might be over.
The other positions are much more able to bounce around the league if they have talent even if there is no opening on the team that drafted them. A running back who shows talent will be picked up by someone else. QB's like Patrick Ramsey a Washington QB's was benched and that basically ended his career. Occasionally a guy who is good enough can recover but what ever happened to Scott Mitchell QB for Detroit or Charlie Batch the man who replaced him?
This is what makes a top QB such a commodity. If you can draft one who can do it good but cross your fingers.