6...Nxd4 is not a good move because of the following reasons.
One of the reasons is tempo. As a general principle, in the openings, you do not want to make your opponent develop his pieces faster than you can. Here, Nxd4 merely speeds up White's development, since White doesn't have to "waste" a move developing his queen. His queen is already well placed on d4.
[FEN "r1bqkb1r/pp2pppp/3p1n2/6B1/3QP3/2N5/PPP2PPP/R3KB1R b KQkq - 0 7"]
Now consider the fact that Black plays 6...e6 instead of 6...Nxd4. Normally, in the Rauzer attack, White has to develop his queen on d2 to support the knight on d4 and castle queenside. Now, consider that after 7. Qd2, Black takes on d4. This position occurs -
[FEN "r1bqkb1r/pp3ppp/3ppn2/6B1/3QP3/2N5/PPP2PPP/R3KB1R b KQkq - 0 8"]
As you can see, by just waiting one extra move, Black reached the same position, but having made an extra useful move of his own - pawn to e6. So here, Black didn't loose a tempo as in the previous line.
However, despite not losing a tempo, even 7...Nxd4 might not be optimal. The reason is that the end result (of exchanging the knights) can be achieved under more favorable circumstances. This is often called "maintaining the tension" in chess. You want to maintain the tension (possibility of capturing the knight) as far as possible in order to release it at the right moment.
I will show you a popular line in which Black achieves the end result of exchanging the knights in more favorable circumstances than 7...Nxd4.
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 Nc6 6. Bg5 e6 7. Qd2 a6
8. O-O-O Bd7 9. f4 b5 10. Bxf6 gxf6 11. Kb1 Qb6 12. Nxc6 Bxc6 *
If you see the chess openings explorer statistics, in this position, White wins this position
36.8% of the time, and Black wins
35.3% as of date.
If you compare these results with the results after 7...Nxd4, the results are quite drastically different. White still wins
36.8%, but Black barely wins
5.3% of the time as of date (I have to admit not as many games have been played in this variation to get a reasonable statistic, but the fact that not many players have chosen this line as early as move 7 ought to give you a pause and make you think it's not an optimal move).