Did you read the page I linked to?
"... the increase in dishwasher efficiency has, in the past, resulted in longer cycle times, as machines are forced to stretch less heated water over longer periods to get dishes clean. Following the last Energy Star specification update in August 2009, a few of the models we tested required three hours per load."
It's a simple trade-off for heating efficiency... if you heat less, but wait longer for the heat to be absorbed over time, you'll be more energy efficient, but take longer to finish.
It's not a big mystery...think of driving your car. If you accelerate quickly and drive faster, you'll get places faster, but use more fuel. If you want to be more "efficient" in your fuel use, one way is to end up getting places slower and being less time efficient. That's the simplest way for manufacturer's to meet energy star standards without any fundamental changes in how their product works.