Between The Olympics explains:
Since the upgrading of the double tuck from C to D, it has become more and more common. It has trickled into the routines of gymnasts who are certainly capable of more difficult tumbling passes. …
If you want a D-part, the double tuck is less deductible than other skills. For example:
… Viktoria Komova had been competing a full in as her second pass since 2007. But recently, she has been playing it safe with a double tuck. I think it is a smart move, and that she should probably keep the double tuck. As you can see, the landing on her double tuck would be defined as “good position.” …
Click through for the detailed explanation. Makes sense.
For most gymnasts the double tuck is safer, too.
Gymnastics Coaching.com 