So with good play 6.Nxf7 is inferrior to 6.d4, but after 6.d4 will follow 6...Be6! 7.O-O! with slightly better play for white. (see the comments of Stefan Buecker)
And this is not about the Traxler, but as pfren said both 5.Bxf7! and maybe 5.d4!? too will be better for white.
I remember watching 2 engines play against each other and after about 20-25 moves black was up 1 knight vs 2 white pawns. Having only 1 pawn advantage. Obviously if your up 1 minor piece the computer should be reading +3 etc. In this situation white had some dynamic compensation which black has to play very carefully as well as white.
So because of these above reasons most black players decided to do something esle. They didn't want to devote all there time messing around with the above lines. They figured it is not worth all the effort. After all when you think about it these lines don't get played often in middle level and higher level chess. So why spend so much effort trying to memorize and study all of these continuations when you don't see it often.
Which is why they play 5...Na5.
It is a more simple approach.
I strongly believe 5...Na5 is the best move.
Yes, for sure 5...Na5 is the strongest move, but when white only know he has to play (after 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd5 Nxd5) 6.Nxf7 Kxf7 7.Qf3+, and he doesn't know how to continue the attack, black will surely have chances to draw (at least). And secondly, the link in my previous reply (http://www.chesspub.com/cgi-bin/chess/YaBB.pl?num=1269280757) will show you, there is no clear theoratical win and that white has to know very carefully what he is doing. I will give a sample line.
And after 6.d4 Be6 7.O-O white is just slightly better.
FOR WHITE