If you back the airplane in, you have to get the tail past the jetway to get to the aft hatch. This probably means that the jetway has to come farther out into the "courtyard" (for lack of a better term) *AND* the extending part of the jetway, that mates with the airplane, has to be significantly longer.

At one point, WN was experimenting with dual jetways at Love Field, to use both hatches. The second jetway had to go up, to go over the wing, and back down to the hatch. I stopped flying WN before I heard about the results. (I was only flying WN between DAL and AUS. At the beginning of 2001, I fell into a deal on a very-low-mileage 1999 Corvette, and driving between Dallas and Austin suddenly became enjoyable enough that I stopped flying that route.)