There are few towers in the country that deal with as much traffic as Centennial does. It's a Level 8 on the FAA ATC facility scale, which is rather rare for standalone ATCTs (those without a TRACON in the facility). The traffic alone generates more opportunity for errors than a new controller might experience at a Level 4-7, where most new controllers go.
There are periods at every tower where they have a lot of trainees, and then there are periods where there are none. For those in the FAA western region, now is the period of rampant on-the-job training. Those places are so full that they won't accept any new hires from the academy.