The 50 ohms question has been probably answered a dozen of times at edaboard. You may want to search the forum for previous contributions, it's a true series topic.
There are several reasons of course, a simple one is sufficient to my opinion: These impedances can be best achieved in coaxial respectively differential pair cables. And yes, they can be easily implemented on a PC board, 100 ohms differential is most easy, 50 ohms single ended needs a somewhat unconvenient trace width on a standard two layer FR4 board, but still feasible.