Why is should be dxc6 instead of bxc6?

General principles tells us to capture, towards the center. However:

The better move is dx instead of bx. I think there are other examples of a bishop takes knight, be it from white or black, and the captures is towards the border instead of the center.

But why should it be dx instead of bx? I still don't get it

Because the rapid development of the Bc8 is more beneficial than the central control achieved by bxc6. 

It sounds right, you've put it very well

"... [After 1 e4 e5 2 Nf3 Nc6 3 Bb5 a6 4 Bxc6, no] one plays [the bad move, 4...bxc6,] since after 5 0-0 d6 6 d4 White is a whole tempo up on the Steinitz Deferred [(1 e4 e5 2 Nf3 Nc5 3 Bb5 a6 4 Ba4 d6 5 Bxc6+ bxc6 6 d4)]. …" - IM Sam Collins (2005)

Another reason is the nature of the imbalances.

By playing Bxc6, White avoids losing a move and gets the better Pawn structure, but Black gets the two Bishops vs Bishop and Knight.

But the Bishop pair is advantageous only in open positions with unbalanced Pawn structures. So it's logical for Black to capture with the d-Pawn, since this leads to a more open position (emphasizing the Bishop pair) than bxc6 does.