The 50 ohm standard is related to efficient geometry for coax cable.
There is a sweet spot where the various losses tend to balance out at a minimum.
Common to see both 50 Ohm(RF) and 75 Ohm for tv and video
For balanced transmission lines (aka twin lead) 300 ohm is common which (may) in part explains 75 Ohm for video since one simple balun design results in a 4:1 ratio.