Sources?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.
THIS. ^^^The reason used to be the availability of of RF devices and current consumption and nothing to do with SAR.
When I started aligning and repairing radios almost 40yrs ago there was no SAR and nobody had a clue on RF safety. Going back in time it was a little less efficient to make power at UHF than VHF and Ni-Cad batteries were the best thing available to power hand helds.
Now we have more efficient PAs and higher capacity batteries so its possible to produce more power on UHF but SAR may have entered the mix late in the game.