Because one of two things will have to happen [Edit; Maybe both!]:
1. An extra hour of (two) sitcoms will be inserted into the NBC schedule after football ends.
2. Either Parks & Recreation OR Community will not be on NBC's spring schedule.
I have no idea how to handicap these two possibilities. Community hasn't done well, but compared to the rest of NBC's shows it hasn't done terribly. I have no knowledge of any change in status for Parks & Recreation or other NBC mid-season shows. And while NBC hasn't had 30 minute sitcoms elsewhere on its schedule, it certainly looks to have a number of holes to fill come spring.
Your guess is as good as mine.