It's a two-way street. If the org (UFC especially) doesn't go along with a demand for for a single fight contract, what then? These fighters aren't going to refuse a chance to fight in the biggest show. It seems that your premise is based on all promotions being interchangeable. In the real world, this is not the case. Some promoters are MUCH better at promotion, and have MUCH better name recognition. Both of these facets factor heavily in the amount of pay and exposure a fighter receives.
You say (later in the thread) that this would benefit both the fighters and the fans.
The fans: One-and done contracts would UNDOUBTEDLY lead to pissing matches between promoters, creating all sorts of problems getting fights signed. Anyone remember the fun we all had waiting for Chuck vs Wandy back during the Pride days? Also, this would make it exponentially easier for fighters to avoid the matchups that they fear most. Good for the fighter, bad for the fans.
The fighters: Say a good fighter has 2 extremely bad performances in a row? Not only will they be offered less than they would from the org that they lost in (that's IF the org decides to sign them for another fight at all), the other orgs would be compelled to pay them less for their next fight as well, as they know that their value has gone down. This is compared to, say, a 6 fight contract, where more than likely, they will be fighting for the same org, and prolly without a pay cut.