Messi is far more influential athlete than Lebron James. Football is miles more popular than basketball, I don't know anyone who watches or even likes basketball, I don't even know who Lebron James is or who he plays for I just heard his name mentioned when I was watching NASCAR.
There are bound to be millions of kids playing football dreaming to be like Messi and he reaches a much larger audience. Outside of America basketball is not a major sport, Football is by far the biggest in the world.
Those Forbes lists are rubbish, they just print these lists so people buy their magazine, I think they just make the numbers up.