Most people automatically assume that because someone plays football they would also have the type of pain tolerance and temperment for boxing. The violence and type of contact in football is completely different from that in boxing. First off the collisions in football is like a car crash. One big moment of impact then you rest for 30 seconds and do it again. Boxing on the other hand is a non stop attack on vital areas that is not protected by $5,000 worth of equipment. In football if you become tired but there is rest in between plays. Boxing on the other hand doesn't afford such breaks. Aside from 1 minute in between rounds there is no time to regroup and catch your breath. The contact is constant and consistent. Football players are tough but the type of pain tolerance required is different from what is required in boxing. As far as temperment football players try to hit as hard as they can on every play. In boxing you can't load up every punch for a knockout. You must have controlled aggression.
Many also assume because football players have big muscles they will be strong and able to knock people out. The problem is huge muscles are more often counter productive for boxing. Bigger muscles tend to tire more quickly and restrict movment. Ideally you want long wiry muscles which are more elastic and more capable of moving rapidly for longer durations of time. This is one reason why I'd rather convert a basketball into a boxer, than a football player. Another reason would be a basketball players required movements are much like that in the ring. A basketball defensive slide mimics movement in Boxing. You have to drive from your hips, not cross over your feet. A pivot in basketball is relatively the same as pivoting in the boxing ring. Not to mention a basketball player generally has decent conditioning.
Due to body type, movements, and general conditioning basketball players generally can pick up the basics of boxing faster than football players.