Aleppo Battle: Why are Rebel Forces losing in Aleppo?Watch
Are Government forces using chemical weapons?
What are Europe's, America's and Asia's reaction?
They are under siege because of several army offensives that have all been backed by Russian air power. Those offensives all involved the better units of the Syrian army, rather than poorly trained conscripts and militias - the government have really focused their efforts and resources on Aleppo because it is such an important target. Russian air power has put the rebels on the defensive for a while now.
The rebels outsie the city have tried to break the siege twice. The first time they reached the besieged rebels but then got driven back, and the second time they didn't even get anywhere near. So the rebels inside Aleppo are pretty much on their own.
There has also been rebel infighting in the besieged rebel held parts of Aleppo, which of course uses up manpower and resources that cannot be replaced. That won't be helping the rebels much.