NASCAR started working with the size of the field a number of years ago. At one point, the size of the field was determined by the number of pit stalls. Later, NASCAR decided to standardize the number for all race tracks. They settled on 36 qualifiers and 6 provisionals making a field of 42 cars. The 43rd position was included when Richard Petty was having trouble making the starting lineup. NASCAR created a "Champions Provisional" to get Petty into the field. Today this provisional is used to help any present or past champion make the 43-car field. If a past champion doesn't use the provisional, it goes to the car owner with the highest points that didn't qualify on speed. NASCAR has had a 43-car field ever since and I doubt it will ever change