Because the schedule is impossible. There's no way in hell I can qualify at (what becomes) the middle of my work day (I work Mon-Fri, 9-5:30). However, for this it seems more able for me to compete as Race and Quali are both on weekend, although there's other commitments (Refereeing and wedding planning) that usually occupy that time.
Trust me, I'd race in more events if I could, but the Qualifying for all of them are at a time that I absolutely can't make (without being irresponsible and taking sick days).