I wondered about this too. Perhaps the sensor readout circuitry didn't allow it, or they had problems with heat dissipation in the smaller body? Not trying to make excuses for Canon, but in all honesty, I'm surprised that it has 4K at all, LOL!
WOW. I must say, I am impressed.
I wish they would have announced a kit with the rumored RF 24-240 rather than the adapted EF 24-105 STM, but at this pricing I really have no complaints. I would pick the RF 24-105 anyway.