White's Response to the Norwegian Defense In the Norwegian Defense of the Ruy Lopez

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1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 b5 5. Bb3 Na5 6. O-O (6. d4 exd4 7. Nxd4 c5? 8. Bd5!)


Neil MacDonald suggests 6. O-O as a response. Why isn't 6. d4 suggested in the book?

Edit: 6... exd4 7. Nxd4 c5? does not work because of 8. Bd5! when White enjoys a comfortable lead in development. I ask what are the repercussions behind playing 6. d4 possibly followed by 7. O-O. Is this variation sharper? 

The author suggested 6. O-O as a solid choice compared to the sharper 6. d4, but I don't see what's so sharp about it. White is simply challenging Black in the center. I guess it gives less information away about the central pawn structure. Is that the answer?
If you choose 6.d4 you must consider it either as a gambit or as a prelude to a queen sortie, since after 6...exd4 7.Nxd4?! Bb7 [not 7...c5? 8.Bd5!] 8.Nc3? c5! followed by c4 traps the Bb3.

7.Qxd4 is possible, but after 7...Bb7 you have to take 8...c5 into account again. (the immediate 7...c5? doesn't work because of 8.Qd5.) White queen will probably lose some tempi, and in the long run the open center may favour Black's bishop pair. For instance, 8.0-0 c5 9.Qe5+ Qe7 10.Qxe7 Nxe7 (10...Bxe7 12.Bd5!?) looks like Black is grabbing a pawn.

[fen "r1bqkbnr/2pp1ppp/p7/np2p3/4P3/1B3N2/PPPP1PPP/RNBQK2R w KQkq - 0 1"]

1. d4 exd4 2. Nxd4?! (2. Qxd4 Bb7 (2...c5? 3. Qd5) 3.O-O c5 4.Qe5+ Qe7 5.Qxe7 Nxe7 (5...Bxe7 6.Bd5!?)) Bb7 (2...c5? 3. Bd5!) 3. Nc3? c5! 4. null c4


In case of 8.O-O, 8.c3 or other moves, you would have to demonstrate what the idea behind the gambit is.