One of the primary reasons is that vehicles can be deployed to when and where the demand is located. Scheduling is improved. Vehicle dispatch is not dependent on driver work rules. Labor savings are obvious. Safety is better also.
Automation improves the economics for large vehicle systems.
PRT can take even more advantage of automation. As vehicles become smaller there is no increase in cost for drivers. It is the increase in the cost of drivers that has conventional transit trapped in large vehicles. The fact that PRT systems will require moving empty vehicles at first seems like a waste. A closer look shows that it is actually the most efficient way to bring passenger and vehicle together.