It's mainly because, on most routes and most aircraft types, the 12,500-mile coach award is the best value for a domestic flight. It is often the first thing to book up.

After that, the next-best option is the 25k F award. You may be thinking "domestic F isn't worth it", but the option looks better if the coach seat is 20k or 30k - or a high paid fare.

Often my domestic AA search will go like this:

(1) Search for cash prices. Ouch, it's a peak date, they're high.
(2) Search for Y awards. Either nothing's there at 12.5k, or maybe the 6AM flight is there.
(3) Search for F awards at 25k. Now I have 3 or 4 options and a couple of connecting cities to pick from.
(4) Decide between a cash seat (usually in Y, perhaps using Thank You Points to pay) or the best 25k F option.

Disclaimer: I'm usually starting at MCI, so I never have a transcon in the mix. On those flights, one might argue that the 25k F award is actually the more desirable seat than the 12.5k coach award. I'm coming from the perspective of RJs and narrowbodies...I'd rather spend fewer miles and be in the back. I'm not sure if my common availability scenario (Y tougher to get than F) holds true on SFO-WAS.