50ohm and 75ohm standards are definded by coaxial background. In coaxial cable when Zc=75ohm there are minimum insertion loss. Maximum transmited power can be achived when coaxial Zc is about 33ohm. 50ohm system is compromise between minimum insertion loss and maximum power.
In TV cable systems we don't have big power, so minimum loss is important. That's why in TV cable systems coaxials Zc=75Ohm.