The short answer - SAR. It has to do with the Specific Absorption Rate of the RF in the human body. RF causes heating of the tissues in the human body. The higher the frequency, the faster the tissue absorbs the RF, the faster the tissue heats. That's how microwave ovens work, and also why it it NOT A GOOD IDEA to stand in front of a microwave antenna, unless you want to get cooked!! So, manufacturers limit the power output at UHF to keep from frying your brain over the long term (aka liability).
Just go online and search for RF SAR for more information.