Saudi Arabia is so desperate to rid their region of Syria's dictatorial regime that they offered Bashar al-Assad's biggest benefactor $15 billion and protection for their vital oil business. The Russians said "no."
According to a report from Reuters, Prince Bandar bin Sultan (pictured above in 2008) met personally with Russian President Vladimir Putin in an attempt to persuade him to stop blocking the U.N. Security Council and withdraw his financial and military support from the Assad regime. In exchange, the Prince offered assurances that the next leader in Damascus would be under Saudi control, and much friendlier to Russian interests than the Islamic militants threatening to take over the country. They'd also start buying Russian tanks again, re-starting roughly $15 billion in weapons contracts between Riyadh and Moscow that have been halted for several years.
Putin however, remains firm in his support for Assad, in part because he doesn't trust that the Saudis can guarantee a friendly regime in Damascus. And also because Russia has never had more influence in the region that it does right now. Nearly every one of Assad's neighbors wants him gone, but most have not been as friendly to Russia in the past. The loose alliance forged between Syria, Russia, and Iran is good for Putin politically and economically, even though (or because?) it enrages the West so greatly.
Even with promises of a friendlier Saudi Arabia pulling the strings in Syria — which some people believe includes the United States pulling the strings in Riyadh — it seems Moscow prefers to stick with the devil they know.