I think there is more money in golf. Meaning that sponsors are willing to bring out the big bucks because they know they can target a certain demographic with it (rich white old guys with lots of money). This translates to lots of money in sponsorship, meaning more TV coverage.
Tennis has a more mixed demographic. Although it has been called an "elite" sport, in reality you have a mixed bag watching it; meaning demographic targeting is much harder for firms.
But great question!