Why, you ask? Well, it's been asked before.
The stratosphere warms and cools radiatively. It cools by radiating heat into space. It warms by receiving longwave radiation from the Earth's surface. If that radiation is trapped near the surface (which is what the greenhouse effect in the atmosphere does) then the radiation doesn't reach the stratosphere and the stratosphere cools. So if an increasing amount of LW radiation is being trapped, there should be a cooling signal in the stratosphere.
And there is. Voila.
And there is. Voila.
Nice theory, unfortunately the tropospheric data does not support what your attribution of the cause.
Nor does the theory take into account other reasons for global climate changes that overwhelm any manmade influences by more than an order of magnitude.
On those cycles we are due for the onset of another glaciation totally unrelated to any of the "global warming or global cooling" theories of the environmental opportunists.