Let union firebrand Justine Bateman explain why (from Variety):

Several SAG board members, such as Frances Fisher, have noted that the WGA's taking the heat for what should have been SAG's battle. They're still perturbed over the 2004 decision by SAG's national board, led by then-president Melissa Gilbert, to agree to a one-year extension of its deal -- which delinked SAG's expiration date from the WGA's and placed it in sync with the DGA's.

"Ultimately, this is our strike," she wrote. "If the WGA had not called this strike, we would all be participating in massive stockpiling. Then, in June, when our contract expires, the AMPTP would have offered us the no-gains-play-your-work-for-free-and-how-bout-some-rollbacks offer we're now familiar with. And SAG would have to go on strike, but because of the stockpiling, we would have no leverage."