A number of new processors either takes multiple supply voltages (one for the core to keep down power consumption, and one for the GPIO to let them interface with other hardware) or have a voltage regulator internally in the processor for the core voltage.

5V core voltage really is ancient technology requiring quite large feature size for isolation. And larger features means larger capacitances. And larger supply voltages means the capacitances needs to be charged to higher voltages. So much higher power consumption for each gate toggling. And more power means higher temperature.