So I am currently working through Chess Fundamentals by Capablanca and he said something I find opaque: following 1.e4, e5 2.Nf3, Nc6 3.Nc3, Nf6 4.Bb5, Bb4 "5.0-0 an indirect way of preventing 5...Bxc3, which more experience or more study will show to be bad."

I have been trying to figure out why, if for whatever reason white does not castle (lets say 5.d3), then why is taking better for black? I looked all over for ways to exploit the king in the center, or ways to take away the white king's castling rights, but I have found nothing.