Two things:
1) It enables the event planner to run a tighter ship, in cases of both the foreseen and unforeseen. Unlimited fields would be much harder to predict to keep everyone happy, and in the case of national events, the priority is the pro schedule. Limiting the sportsman classes, who are largely considered by event staff a necessary evil and by fans as uninteresting filler, keeps everything more streamlined and limits potential of non-pro delays. Also, with personal rigs getting bigger and bigger, pit space is now at more of a premium than it used to be. This is a bigger issue at some tracks than others.
2) It encourages participation in the divisionals. Entry to national events is staggered with those having more grade points having earlier access. If the quota is met by the time your grade point level opens, too bad. Your only chance then is to snag a spot if someone withdraws. Since you get a grade point for each divisional you run (as well as a few other specialty events), it's to your advantage to make as many as possible if your goal is to run nationals.
