Tennis and golf are obviously very lucrative if you get to the top but the initial outlay to begin playing, like purchasing equipment, membership fees, travelling, etc, are ridiculously high to begin with and it is quite difficult to get started, whereas with bodybuilding it can be as cheap as you want to to be - all you need is a gym membership and you're set. The level of commitment, dedication and preparation required to get to the top in my opinion is pretty much the same for all sports. A pro bodybuilder is just as dedicated to their profession as a professional tennis player is and vice versa so that factor shouldn't really come into it.
In the UK, bodybuilding, powerlifting, and strongman are all working class pursuits so people who aren't that well off do tend to gravitate towards them (as well as football of course). Only rich people play tennis and golf.
