It's so interesting that for people who suposedly revere the Holy Scriptures as we evangelicals alledgedly do, we take such a casual attitude in our approach to them. Part of the problem is that we don't want to look catholic- now I would say "we don't want to look Orthodox", but not many of us are aware of Orthodoxy. It's part of a larger problem in a sense: Reverence= Sterness= a Church that doesn't appeal to the mythical "Everyday man" that Fr Schmeman spoke of. Thus our reverence towards anything is carefully measured in our Church services that begin to take on the appearance of a pep rally: As A.W. Tozer said, the program becomes more important than the Presence.
Thus, while we're so zealous to make sure that scripture is understood, we're equally wary to look formal- (as if that draws in meaningful numbers thru our doors). Still, that doesn't quite get to your question of why this lack of holding up the Gospels in any kind of pre-eminence. I can only putter around and think perhaps that 1. We love the intellectual appeal of the epistles, they have a beautiful way of applying Scripture to our lives- they're not dealing so much with the mystery as they are application- we like propositional arguments. I still love the epistles for these reasons.
2. Perhaps, just perhaps, the Gospels- while absolutely loved and cherished- show the most amazing mind-boggling mystery that mocks the intellect: the Incarnation. We don't like mystery. We believe in it, of course, but it doesn't unravel itself as neatly as say a Pauline Epistle. I suspect that this second possibility is not so much an active choice on our part, as much as it is passive. We then like to say we teach, affirm, and emphasize all of Scripture equally; but I'm not so sure that's actually and accurately borne out.