By definition a reversible cycle must have an infinitesimal temperature difference. That means ($T_{high}-T_{low}$) is very very small. Any finite temperature difference would increase the degree of irreversibility, hence reducing efficiency.
A Carnot Cycle has efficiency $n=1-\frac {T_{low}}{T_{high}}$). If ($T_{high}-T_{low}$) is very very small, as said above, then ($\frac {T_{low}}{T_{high}}$) must be close to 1. That would make efficiency $n$ close to zero. But Carnot efficiency is the maximum achievable efficiency for any heat engine. That requires the ($\frac {T_{low}}{T_{high}}$) to be as low as possible.