Iran and the Middle East as a whole is not a core interest for the Russians.
Russia will try to promote and defend Russian interests in the ME, but they are not prepared to 'bet the farm' on it the way they would have been over Ukraine, Georgia or other places far closer to home for them.
I think the west in general, and America in particular, made a serious of calamitous mistakes in the lead up to Syria.
First they way overplayed their hand and made the Russians bare their teeth first in Georgia, and then in Ukraine.
Having had to 'blink' repeatedly and back down, the US Obama administration lost the plot a little, and got into the habit of blinking and backing down before the Russians.
The initial Russian Syrian deployment was very limited and temporary. You got the impression they were ready to pull up stakes and be gone on very short notice.
That was, in my view, the Russians dipping their toe and testing the waters.
The lukewarm western response to that deployment, and Turkey's epic fail blunder ambushing that Su24 and the feeble and divided NATO response gave Russia the confidence that NATO was not prepared to fight them over Syria.
The Iran S300 deal is an ace card the Russians are holding on to, to play when it suits them.
It is a carrot they could dangle in front of the Iranians, and a giant stick hanging over the heads of the likes of Saudi Arabia and the US.
However, because of how powerful that card is, if the Russians played in, they can expect significant push back and responses from the likes of the Gulf States and America. Even currently friendly powers like Israel will be forced to jump in and counter such a deal, which may throw their entire Syrian game plan into chaos.
Simply put, I think the Russian Syrian deployment is going as well as Putin could have hoped for going in. As such, he is content to let that play out rather than make any major new moves that might derail the progress they made in Syria.