Electronic Counter Measures (ECM) (either Iranian or Russian) most likely took out the "smart" AGM-158B JASSM-ER missiles used by the American's. Now the Russian have the details of these missiles to add to their already effective air defense systems (they got the information about tomahawks decades ago). I strongly doubt any of the old soviet air defenses took out those missiles, just the tomahawks and its UK variants that are a very crude missiles in terms of speed, accuracy and capabilities.
The Syrians most likely had the S300 already on the ground but were never activated, now it seems that they will have the S300 in spades and more effective radars. Their is no way the Israeli's could take these air defense network's down and the ridiculous claim they took out most of the Syrian air defense network in past strikes makes them look even more stupid. Plus there is the possibility the S300 maybe part manned by Russian's which means Bibsy will be on the phone to Putin all week to try to avoid this possibility of hitting Russian servicemen without creating WW3.
The strong ability the AGM-158B's have to change directions and targets at a higher speed to the slow tomahawks would have been used to light up the locations and capabilities by exposing the Syrian air defense network to thoroughly map it out for further strikes either by the US or Israel. The fact they never activated the Russian S400 let alone S300 or S350 means it is a massive loss to the war mongers. All they can do is hope their own ECM capabilities are more superior to the Russians and Iranians as an alternative to shut the air defense network down. You can bet the stuff being flown in cargo planes every day by the Russians will be to negate this option.