Because she wants time to make a nuclear agreement work:

Sen. Patty Murray (D-Wash.), a member of the Senate leadership, came out Wednesday against passing a bill to slap more sanctions on Iran during an interim six-month nuclear deal between that country and the world powers.

"I believe the Administration should be given time to negotiate a strong verifiable comprehensive agreement," Murray said in a letter to her constituents. "However, if Iran does not agree to a comprehensive agreement that is acceptable, or if Iran does not abide by the terms of the interim agreement, I will work with my colleagues to swiftly enact sanctions in order to increase pressure on the Iranian regime."

It's a bit of a change for Murray, and it makes it less likely new sanctions will go anywhere in the Senate.