I'm no expert here, but there is a difference. Lets put it in context. So for individual sports, say tennis, golf etc, the competitors are self employed and make their livelihood through prize money at tournaments. Cyclists are essentially salaried employees and make their livelihood as part of that employment. Footballers too are essentially salaried and as such I guess the prize fund for winning a tournament, eg champions league, world cup etc I would expect is also relatively low.
Like I said, I'm not in the know and will gladly be corrected and also agree that cyclists' salary levels themselves are low compared to say football. But remember just as many a domestique is poorly paid, there will be many tennis/golf pros at the lower end who also arent earning grand slam prize funds.