How do I explain why the Tibetan plateau is colder than lowlands at similar latitudes?
The answer is because the Earth is not a static system.

Due to the ideal gas law, air cools as it rises.  This is referred to as the dry adiabatic lapse rate.  However, you are curious why every location on earth is not the same temperature at the same latitude.  We know this is not true. But why is not true?  Weather.

The earth, as with most natural phenomenon, attempts to achieve static equilibrium.  The differences in incoming and outgoing radiation leave the earth with net energy reserves that need to be balanced out.  Weather is the mechanism that the Earth uses to attempt to achieve balance. 

Okay, so we know that weather is disrupting static equilibrium on earth.  But how does that work?  Wind.  Or better yet, advection.  Advection is simply the transport of an atmospheric parameter (such as moisture, temperature, or rotation) from one location to another.  So, wind can 'transport' temperature, moisture, and rotation.  

You don't necessarily need a storm to advect any of these parameters. In fact, they are always in motion.  Cold air is always moving away from the poles.

Still, that doesn't quite answer it, does it?  Well, imagine you are hundreds or thousands of miles away from Tibet.  You aren't in a mountainous region anymore but instead a flatlands at sea level. The environmental lapse rate is 6.5°C/1000m above this location (because we are not dealing with an air parcel).  Tibet is 4500m - 8850m high.   That would be around approximately 32.5°C of cooling at 5000m.  The air that advects into Tibet from elsewhere is on average of 32.5-50°C cooler than at sea level. 

So, it isn't so much that Tibet is receiving less radiation than other points at the same latitude (its not). Its more that, the air at other locations at the elevation of Tibet is much cooler than the air at sea level. Remember, we measure air temperature!