Let me sum it up for you using a comparison:
Popularity: Bolt is one of the biggest stars in track history - the track community obv nows him and he managed to be known by the wider sports public as well via his Olympic and world record conquests. Bale is a top 5 player in the world at best, only futbol fans really know him (not like Messi and Ronaldo).
Considering the above and the earning differential (Bolt earns peanuts compared to what Bale is pulling and going to pull) -- the factor that matters is the sport they are in:
Track - boils down to being an Olympic sport, otherwise the masses are never watching. Yes it takes a tremendous amount of hard work to be the world's best but so what? Is it entertaining to non-runners? Is it a sport that many non-runners can really relate to? Do we see interesting outcomes and dramatic endings or legendary rivalries?
Futbol - people are born fans through family-following. Most fans never played organized futbol (like NFL). There are great match ups every week/month, it's competitive all year round. The game play is entertaining, there are no long pauses or commercials to distract you.. There is nothing like watching the last 20min of a final when the teams are tied or favorite is down a goal or two. The sport is full of comebacks, interesting developments and a lot of drama.
Lets be serious, track is extremely boring... all this coverage, interviewing and explaining going on just to watch a couple of false starts and a 10sec race with mostly predictable outcomes... there's only so much you can do with a sport like that...hence there will NEVER be as much money in it... so please stop the pointless comparisons. If Bolt had even a slight chance of being a pro football, trust me, he'd be spending his chump change training 24/7 for that instead. That's what makes other sports harder (believe it or not).. it doesn't just take speed, it takes strength, good judgment, balance, skills, coordination, mental toughness... so many more variables than is visible in track. TRACK IS BORING. I like it and find it interesting, but you better believe if I had to chose I would be watching the world cup final instead of an Olympic 100 final. Try following other sports for a bit and you'll see what I mean, it will then be pretty obvious why there's a giant gap in earning potential.
As a final note - from a sponsor's perspective.. who would rather put your ad money on -- a futboler who will be filmed for 90+ minutes using your gear with an audience in the 10s of millions or a runner who will be on tv for at most 10min with maybe 50k tv viewers? That should tell you everything.