Liukin had limited D / E tumbling options on floor, in part due to the much stricter requirements for receiving credit for twisting elements beginning with the 2006-2008 COP. In the previous COP, you could be short by up to 90 degrees and still receive DV (although you would receive a 0.2 deduction for being that underrotated). Starting with the open-ended Code, all twisting elements must be fully rotated to receive credit. Throughout her career, Nastia underrotated her Randi, and she started having difficulty fully rotating her triple twist in 2006 as well. For someone also including a layout front double full and a 2 1/2 twist in her routine, this meant that these skills were at high risk for ending up as repeated elements and also potentially losing CV and CR/EGR credit.
We never saw enough double salto work from Nastia to tell if a full-in or an Arabian double front were viable options, but I would be surprised if the execution would have been much better than what we saw on the double front. Also, with a double tuck only being a C at the time, it made sense to take a chance on the double front instead.