I'm curious about this lesson

I just was in this lesson and this move is being shown as incorrect.  The object is to protect the C7 pawn. Obviously, the Knight (Nb5) is looking to capture it. As you can see, my move was to directly challenge the knight with the bishop (Bd7).  This kind of a challenge would actually protect the pawn, as the knight would either back up or be capture by that Bishop. 

Yes I figured out the answer and I agree that it makes sense too, but why would my response not act the same way?

Bd7 doesn't accomplish anything. It doesn't protect c7. White can respond and play Nxc7 which forks your two rooks.

I'm going to leave this up, but I will state I was and still am pretty tired and wasn't thinking right. I got locked into "my bishop is able to attack the night". While the angle is right, the Knight's next move would be to get the pawn anyway, so yes, @HeckinSprout is right, my bishop wouldn't be able to take that piece or prevent the capture of the pawn, so yes, it's useless.

Thank you HeckinSprout!