There is a technical reason why HW2 can't support the dashcam, but there hasn't been any indication of why HW2 doesn't support Sentry Mode.
But, the primary SOC along with the discrete GPU isn't used for AP while it's parked. So it should be free to use for Sentry mode.
I think they are using the AP SOC/GPU to process a Sentry Mode NN. Then the other SOC is used to process/compress/send the cameras feeds to the MCU to be save on the USB drive.
So just like with the dashcam feature the HW2 cars don't have the needed resources.