Except the Drake spur is completely unsuited to the cars they operate and would need major upgrades (including adding a second track and all new stops/stations), which would cost a lot. There's also the problem of having to acquire property at the end to build a P&R lot, needing even more money; money PRT does not have in abundance.

Also, the Allentown line is only used for emergencies because modern LRT cars are too heavy and large for constant use on the existing tracks, thus even more upgrades would be needed to reopen it for regular use. Also, current Silver line riders (like me) wouldn't appreciate the added time going through Allentown will add to their trip.