Because Nxd5 is a bad move! 6.Bd5 Nxd5? 7.exd5, displacing the knight and also threatening d4. 6.Bc4 is good according to Stockfish
I understood, thank you for explications.
So my next question is : Why 6.Bc3 ? (2nd theorical move after 5. ... Ke7) Now 6. ... Na5 exchenge Knigth against Bishop and the bishop doesn't win tempo in d5 square !
Because Nxd5 is a bad move! 6.Bd5 Nxd5? 7.exd5, displacing the knight and also threatening d4. 6.Bc4 is good according to Stockfish