I'd assume it's because the alky classes are still a regional/national structure for their points championship while PM is only national. If PM changed its format to run at divisionals and have both regional and national championships that incorporate races at all levels, they'd probably have similar entry limitations. However, since it started as an exhibition class at nationals only and slowly worked its way toward the current format as a semi-pro class under NHRA's umbrella, I don't see that changing.