Some IRB’s are more conservative than others with respect to the ethics of keeping patients in a placebo arm. INCY itself had no reason to limit the duration of placebo treatment, so it’s reasonable to assume that the IRB’s insisted on the crossover at 12 weeks.
A more interesting question than the one you raised, IMO, is why placebo patients were re-randomized to only the two highest doses of INCB028050 rather than to all three doses.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”