Why does the monitor only use 2.4GHz wifi

Two good reasons. First, 2.4GHz travels further and passes through walls and greenhouses better than 5GHz, which matters when a monitor is out in the garden. Second, the low power chip inside is designed for it. For a sensor that sends tiny readings, 2.4GHz is the right tool, and the range you gain is worth far more than the speed you would not use anyway.