The 1.5kW limit is really per transmission. But there are other limits as well that may be reached first. Some bands have a much lower power limit.
Also, high on that list is safety, and this isn't just a good idea, it's legally required in the amateur radio regulations. Above 50w and certainly above 100w you should be doing an environmental study to make sure that people in the environment of your antenna are not absorbing radiation above safe levels. In a multi-antenna or multi-transmitter situation, the radiation from each adds up to a total that may not be safe. (Note: the FCC recently dropped the amateur radio exemption for low power stations. Now all stations are suppose to do an environmental study.)
Also, high gain antennas can focus power to unsafe levels even when transmitter power is low, and it is important to make sure people can't walk through the beam of such an antenna.
Physics may also limit your power. As your power level increases, SWR and loss in your antenna system become more and more critical. At high power levels, loss translates to heat, and enough heat can burn through components, and SWR becomes a multiplier for these problems. While low power stations try to minimize loss and swr for efficiency reasons (to make best use of power available), high power stations must fix these issues to operate at all.