This is one explanation, from Leo Smith in Tablet Magazine, September 7
…Obama’s inaction in Syria is not simply part of the hangover from the failed American war in Iraq, or of the president’s personal psychology. There is something entirely practical at stake here, too – namely, the Iran deal. The explanation is, in fact, a simple one: U.S. intervention in Syria against Assad would have made the Iran deal impossible. In fact, U.S. support for Iran’s continuing presence in Syria was a precondition of the deal, according to no less an authority than the president himself. In a December press conference, Obama spoke of “respecting” Iranian “equities” in Syria – which, translated into plain English, means leaving Assad alone in order to keep the Iranians happy.