The biggest reasons why there are more big paying late model races than sprint car races:

1) As it was mentioned before, in order to get all the top names in late model racing, you need to pay a bit more as there are multiple sanctions and frankly guys like Bloomquist have been really smart at not racing for less money.

2) I actually think the WoO sprint car series works against having shows that pay more than the normal 10k to win purse as your guaranteed of getting their drivers to your track whether you pay the minimum or pay more. Unless a promoter thinks a higher purse will draw more people, it doesn't make a ton of sense to pay more for the same drivers.