It might be trademarked and disallowed (think careerbuilders, etc)
or nobody may be bidding on it.
more likely the first option if you're asking...

Sounds like noone is bidding on the terms.. whether it be due to trademarks or just noone thought of using the keyword(s)..

1. No one bidding on them. Unlikely but not impossible.
1B. As Chris and WA said, they may be trademarked and can't show up in the ad itself, although there are ways around this and although having the term in your ad can only help, if you are not allowed, I still find it unlikely nobody is bidding on that keyword.
2. No one is bidding because it's an unprofitable keyword.
3. Nobody is bidding at the time you did the search. Is it seasonal? Maybe advertisers have realized that the keyword is more profitable between 1:00 pm and 4:00 pm on a Saturday in November. Maybe all advertisers have exhausted their budget when you did the search.
4. Or simply, as Chris said, no one thought of using that keyword. Yet.