It wasn’t really explained because Dan Wheldon was the only person who actually took up the offer. Indycar was trying to attract drivers from other series to compete in Las Vegas by offering a $5 million prize to any non-regular driver who won the race from the back of the grid. They didn’t get much interest.
I don’t think Wheldon was solely in it for the money. The challenge probably appealed to him, and he promised that if he won, he’d split the money with a fan chosen by lottery.