Why can't phosphorus trifluoride be used as a Lewis acid catalyst in electrophilic substitution of benzene?
The best answer I can give is that there is enough space on $\ce{Sb}$ in $\ce{SbCl3}$ for an additional Lewis Base to interact with a nonbonding orbital on the $\ce{Sb}$ atom. While $\ce{P}$ can be hypervalent, the LUMO in this case would be $\ce{P-F}$ $\sigma^\ast$, which is really high in energy (meaning that it doesn't easily accept electron density from other species in solution).