Money, usually. There is individual prize money for the masters race at the USA champs, but no team money. At clubs, there is only team money. Unless you run for a masters team at the club xc race, there is no point to running in the masters race. Some of the faster masters women do run on open club teams, so they run the open race.
But, interestingly, the masters winner at the USA race did not get the $1000 for first because it was based on "age-grade", not overall place. A 59 year old won the $1000, with a 77 year old getting 2nd. Colleen got $450 for 3rd in the age grade money, followed by a 51 year old and a 61 year old.
She was on a masters team for this race, coached by her husband, I believe, and they won the team title over another Colorado masters club and two So Cal teams. For some reason, the Boulder teams (all 3 of them) generally compete at the USA meet. The team competition is much stiffer in the club race in December, though.
As you pointed out, she would have run faster in the open race as it is much harder to run a fast race when you have no one to race. The money there went 10 deep, although she would have had to run more than a minute faster and beaten Kara Goucher to get as much money as she did in the masters race.
As far as I could tell, there were no really fast masters men who opted for the open race in either the club race in Dec. or in the USA race in Feb.