How do you know? We know that Russian troops have been involved in recent offensives. Are they going to suddenly sit this one out, given that Assad seems to have considered Aleppo (as Syria’s formerly most populous city) as the most important battle so far of the war?
I am saying that Russian air power so far has not been that decisive. Russia has made bombing runs in Aleppo, against other rebels in Idlib and elsewhere in northern Syria, and a few strikes against Da’esh. Outside of Aleppo, which was already hotly contested prior to Russia’s active involvement with no real likelihood of the rebels seizing all of Aleppo, Assad has not been able to pull off many successful (in terms of seizing significant territory) offensives since late 2015 when the Russians started their air campaign.
As to why Syrian rebels could not seize Aleppo and Damascus before Russia got involved if they had U.S. support (but Iraqi military units could seize Tikrit, Ramadi, Fallujah, etc.), that’s easy. The support given to Syrian rebels so far appears to consist of light arms, anti-armor TOW missiles, communications equipment, medical supplies, and training. Maybe some light vehicles. The U.S. has provided Iraq with tanks, armored personnel carriers, humvees, F-16s, and an assortment of other light and heavy weaponry. The U.S. military is embedded much deeper with Iraqi military forces because there is a government and an organization to work with.
Iraq had a shell of a military in late 2014 that was built back up, in conjunction with relying on Shiite militias that the U.S. has not armed but have been effective here and there. The biggest assets Iraq have are the Kurdish Peshmerga and the Iraqi Counterterrorism Forces, which is an elite special forces-equivalent unit that has been specially trained and equipped by the U.S. and has done the bulk of the important fighting and clearing operations so far. They were heavily involved in the Ramadi, Fallujah, and the ongoing Mosul operations. The Iraqi military also has a command center in Baghdad where U.S. and Iraqi forces work together closely to coordinate and plan operations, tactical deployments, requests for air strikes and reinforcements, as well as work with the Iraqi Ministry of Defense. Given the loose and disorganized nature of the various rebel groups in Syria, there is no equivalent centralized force or location where such deep coordination would be possible, let alone the infrastructure and trust needed to provide heavy weaponry such as tanks, air defense weapons, planes, and helicopters to any one or grouping of Syrian rebel forces.
The rebels didn’t just need arms, they needed heavy weaponry - armor, planes, air defense weapons. But most of all they needed organization. No one rebel group, even if you include Da’esh, has the numbers or the organization to defeat Assad alone, even prior to Russian involvement. The U.S. was not going to just provide manpads, tanks, and indefinite ammunition to any rebel group that wanted them, particularly since significant training would be required so that rebels could operate any and all heavy weapons proficiently and effectively.
They weren’t like the Northern Alliance in Afghanistan in October 2001, which had organization, which knew how to operate heavy weapons like (old) tanks, and which had the capability of conducting a mass offensive over a large front. Even then, it took a smattering of U.S. ground forces in addition to sustained air power to “finish” the job of at least kicking the Taliban largely out of Afghanistan and into Pakistan.