Nose-wheel steering explanation
From the preamble to Amdt. 25-42 to Part 25 reads in part:
Several commentators recommended that the proposal be revised to allow the use of nose wheel steering in the determination of VMCG under Sec. 25.149(e), if control is through the rudder pedals and the demonstration is made on a wet runway. The FAA does not agree. The effectiveness of nose wheel steering depends to a large degree on runway friction characteristics and the load on the nose wheel. Certification tests on a wet runway would not cover the more extreme slippery runway conditions or all possible variations in takeoff conditions and techniques likely to occur in service. The FAA therefore believes that VMCG should be determined without the use of nose wheel steering, as stated in proposed Sec. 25.149(e).
FYI -- Explanations to changes to FARs are normally found in the preambles. The preambles can be found at the same link posted above.