This is where karting is different than car racing. Race car track promoters make decisions based on paying spectator numbers and worry about enough coming that night to cover the guaranteed purse money. In karting, since we basically race for our own money (kind of like playing poker which some nights would be more fun) I guess the first consideration would be track points for that night. Do you really expect racers to tow there in spite of a high chance of rain, just so they dont get hurt in points if they do race? And yeah what Brian said about fuel costs.
I guess you have to decide what is going to put a bad taste in the fewest mouths