Founders of channels or anything really want for their product or channel to be successful. Really successful businessmen will conduct research about their product before actually building their company. In this case, the founders of this channel worked in the business of sports prior to the channel. They probably had a lot of knowledge of what people wanted to watch in regards to sports.
Also, there were not as many channels out there in the 1970s as there are today. ESPN because so famous due to having experienced founders, little competition at the time of the creation of this channel and ESPN is so popular because it has withstood the test of time. Other channels have come and go, but ESPN has stayed based on these reasons.