According to the Department of Justice website,

Generally, commutation of sentence is an extraordinary remedy that is rarely granted.

The statistics bear out that it’s easier to get a pardon than a commutation. Clinton  granted 396 of the 2,001 pardon requests he received, but granted only 61 of more than 5,400 requests for clemency. In the previous 12 years of Bush and Reagan, clemency requests were granted only 16 times.

Again, from the DOJ:

Appropriate grounds for considering commutation have traditionally included disparity or undue severity of sentence, critical illness or old age, and meritorious service rendered to the government by the petitioner, e.g., cooperation with investigative or prosecutive efforts that has not been adequately rewarded by other official action.

During his sentencing, defense attorneys made much of Duke’s history of prostate cancer. And the former congressman has cooperated with the investigation, even though prosecutors never called him as a witness at the trial of Brent Wilkes, for reasons that remain unclear.