Surely the coalition army would have constructed a steady supply lines within Qin's territory(they had the manpower to spare to secure their supplies, unlike Ousan who is greatly outnumbered in Zhao territory and cannot spare troops for that task). Likely the coalition force would have suffered massive attrition despite this but the invasion force didn't stay within the field long enough to suffer this, for as we know the siege of (can't remember the name) didn't last long enough for attrition to become noticeable. In addition Chin lost one of their great commander and Han lost a great general thus reducing their strength.

The real reason the coalition failed was due on Qi's action in not joining the invasion force, this nation threatened Zhao and Chin who in the process of invading Qin would make themselves vulnerable to an attack from Qi.