Short answer: the 12V bus in a hybrid isn’t designed for jump starting.
Long answer:
Toyota hybrids have no alternator. The 12V bus comes from a tap on the (very expensive) hybrid inverter unit.
The inverter does not have the same output current of an alternator and can take less abuse in terms of shorting or overloading the output. There is a master fuse on the Prius that will blow if you try to crank a dead vehicle while the Prius is on.