Because of the stage picture you're presented.
If you're right in the very front you'll see the foreground of the stage and those performers very well, but you'll miss out a bit on some of the performers than are a little further back on the stage, as they'll be obscured by those in the fore-ground. (also, several theatres have lighting on the very edge of the stage, which even further obscures the stage-picture.
Right about in the middle of the theatre, you're getting the exact stage picture that the producer of the show intends, and as such, for those sort of seats, you'd expect to pay a premium.
(I've been studying with Second City for about the past two years, and I've had this explained to me by the producers/class teacher on leading up to the first publicly open show we gave).