why turn off truck's air conditioned when going uphill

Many new cars and trucks automatically turn off the AC compressor when needed to reduce the drag on the engine. Are you towing or hauling heavy loads? I'm surprised you can't make it up hills. Most modern trucks have horsepower to spare.

Air conditioning not only takes a fair amount of power to operate (removing power from the wheels) it also dumps a ton of heat at the condenser which is in front of the radiator. When the air conditioning is on, the condenser is hot, so the radiator has to cool off with hot air coming off the condenser.

But, depending on the vehicle, there should be no reason why you have to turn it off to go up a hill. Either something is wrong causing it to overheat, or it was a poor design from the factory.