The situation in Syria as it stands today is complicated, sure. But it's also pretty easy to summarize why Aleppo and its outcome is important in the proxy war.
Aleppo is a major population center and a "hub" in the spoke-and-hub network of cities and towns in Syria. When - and if - there is a political transition for the government of Syria, the forces that stand in Aleppo will have an outsized influence in the outcome.
In this manner Aleppo has been an important battleground for the current administration in Damascus and the Russians who have national security interests in the secular government of Syria. For the same reasons the opposition - the rebel and terrorist forces in Syria - have been trying to take and hold Aleppo.
The United States and Russia had been trying to broker a ceasefire, but ultimately the city's value to proxy forces and foreign backers proved too much and the mutual trust between great powers too little for a negotiation to be reached.
Now that the ceasefire is off the table, the United States does not have a legal option for intervening. It has not ruled out - and may be engaged today in - covert illegal activity with respect to attacks on Syrian Government forces and infrastructure. On the other hand Russia was invited in by a government that is still internationally recognized and can intervene in Aleppo on a legal international basis.
One option open to the United States is to try to intervene in Aleppo on account of a humanitarian basis, in which case it needs public opinion and material support to bolster a legal argument.
This is the reason that the United States and its media companies have been focusing so much on the situation in Aleppo, narrating it as a particularly horrible instance of the civil war.
The same sort of narrative campaign, it is easy to notice, is absent from reporting on Sanaa (Yemen), on the current operations in Mosul, of the Turkish (illegal?) military invasions of Greece and Iraq, of US bombing in Northern Syria, of Kabul and of military operations in Afghanistan, or of the mass innocent casualties in a number of places around the world from the Predator Drone mass assassination programme.
Aleppo is a tragedy. Truly. The proxy war in Syria - and all of the regional and foreign power who have made it worse by pumping weapons and funding into it (including the US) - is a horror, and an acknowledgement that we still aren't at a place with our civilization where we can resolve tensions of competing international interest with rules, peace, diplomacy, and mutual benefit.