who gets to score a touchdown, and who normally doesn't

My wife was amazed to see 330+ pounder BJ Raji waltz into the endzone on Sunday afternoon on an interception return.  Upon learning that it was his first touchdown, she wondered why he's never scored before

BJ Raji's job with the Green Bay Packers is to play defensive lineman.  This means he's normally lined up at the line of scrimmage directly across from the offense.  In most instances, since he is normally quite close the the quarterback, it would be incredibly difficult to catch a football that is coming at him at 40 mph.

In almost all instances on Sunday, Raji's job was to pressure the quarterback or tackle a running back.  On the play in which Raji scored a touchdown, he faked a rush on the quarterback, but then dropped back into coverage against a Bears wide receiver.  Raji found himself directly in line with where the Bear's quarterback Caleb Hanie was trying to throw the ball.  Raji made the catch, and since he outweighed the only person who could catch him, Bears quarterback Hanie by over 100 lbs, he knew he would make it to the end zone.

Perhaps Raji will score another touchdown in his NFL career, but it isn't likely to be bigger than this one.

On the defensive side, you are more likely to see a cornerback, whose job is to cover wide receivers score a defensive touchdown.  They have many more opportunities since the football is often thrown in their direction, and if they do catch an interception (or pick up a fumble), they are often the fastest players on the field, and the toughest players to catch and tackle.