Really it comes down to the number of routes that they can run because they’re in the middle of the field. Whenever a defensive back is making his presnap reads, he can immediately make an educated guess of what routes the receiver will run. When dealing with an outside receiver that read is a lot easier, and the defender also has the sideline to use to his advantage.
But in the middle of the field, there is no sideline to work with and the number of potential routes is a lot higher.
In the Patriots offense, the slot receiver is the chains mover. We saw it with Troy Brown, we saw it with Welker, and we’re seeing it now with Edelman and Amendola. Not to take anything away from any of those players, but if you’re playing that position, you’re going to get a lot of balls thrown your way. The design of the offense just makes it that way.
Now each receiver has their own strengths that the play design can be tweaked to enhance, but ultimately their job is to read the defense, know where the holes in the coverage will be and often be the “hot” receiver should the defense blitz. They need quickness, sure hands, and have to be fearless going over the middle.