According to this link, Russia has cancelled its delivery of S-300 to Syria because of Israel's lobbying.
But, I am curious, What is Israel offering Russian in return?
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News reports in Israel quoted an anonymous source in the Russian defense ministry, as saying it would take months to train the Syrian forces with using these batteries, making them ineffective.
You would think that Russia would stand to lose a great deal of money from cancelling such a deal, but the truth is, Syria is broke, and can't afford paying for these systems anyway. Had they been delivered, it would have been with a really long credit line, so Russia wouldn't have seen the money any time soon anyway.
This is all very new, so I can't find sources in English for this yet. This source, from Israel's public broadcasting corporation (Hebrew), quotes an advisor at the Russian defense ministry, as saying that while Netanyahu asked for the delivery to be canceled, this was only a minor consideration in the decision. The major reason was the time it would take to train the Syrians with the system.
Situation is a bit more difficult. It is a fact, that middle east is just a bunch of tensions between different countries.
Russian position on Israel-Palestinian conflict is simple: saving status quo. Peace on middle east can be reached without genocide on one of sides, only if war stops in all regional countries. And if we are going on a road to peace, it is unacceptable, to sell such powerful systems to one of conflict sides. Why Syria(SAR) in such situation is very important - for now it is a place for direct Iran-Israel confrontation, because there is a civil war on its lands. And it is much more easy to strike or place forces there for both sides.
So, russian forces for now are helping to stop the civil war. And then, after it, there will be no place for Iran forces on Syrian lands. And without such confrontation place, war between Israel and Iran is will be just impossible - they don't have shared border.
Total: this is a step for not-heavily-supporting one of Israel-Arab conflict sides.
PS: TsahiAsher answer also means. This is a very complex systems, this can be another argument for canceling.