NASCAR starts 43 cars in each of its top sanctioned events because it's a rule that came to be through the years because of necessity instead of normal decisions. As can be seen with the second annual Daytona 500, in which 68 cars qualified, a need soon arose to restrict the field. Due both to safety as well as money issues, NASCAR eventually came terms with the starting grid of 43 cars per NASCAR sanctioned event.
Over the course of the years, NASCAR didn't mandate how many drivers would make the field at any particular track, instead they allowed the track to decided, and that would be based on the amount of pit stalls the track had on pit lane. This restricted the number of cars per race, but didn't really regulate the starting grid.
During the 1990's, NASCAR began to come to terms with the necessity of regulating the starting grid, not only to fairly represent NASCAR s best at each track, but to also be fair to the drivers that both made each race and those who didn't.
It was decided that NASCAR maintain a "top 36" which was to be consisted of the top 36 drivers in the points standings. The same points standings that crowned the champion each year. It is now a top 35 in driver's points. These was to assure those drivers who where consistently the best in the sport that they would have a starting position in each race.
Besides the guaranteed top 36, NASCAR decided to expand the field to 42 to allow for 8 other drivers to "race" their way in with the next highest qualifying positions to lock into each race.
Since then, the field was expanded once more to include what is often called the past champion's provisional. This was created because fan favorite and NASCAR legend Richard Petty was having troubles qualifying for the races towards the end of his career. This would assure that he would be in the race each week, whether or not he was able to qualify.
Since the first inception of the "Past Champion's Provisional", NASCAR has tweaked that rule. Now a drive can only use that provisional 6 times per season this started with the 2008 season . The most recent champion that is not already locked into the race due to owner's points, or due to fast qualifying may use the provisional.
Unlike the Indy Car series, which has problems filling it's starting grid of 33 except at the Indy 500, NASCAR never has a problem qualifying all 43 spots. Even during hard economic times, NASCAR's starting grid has seemed to thrive. In this year's Daytona 500, NASCAR saw 56 drivers vie for the 43 spots.
Don't expect to see NASCAR changing the starting grid anytime soon. It's now a part of NASCAR tradition. A sport which runs and thrives on it's deep rooted tradition.