Question

Why if Altitude increases then temperature decreases?

Answer 1

reason is that the troposphere's gases absorb very little of the incoming solar radiation. Instead, the ground absorbs this radiation and then heats the tropospheric air by conduction and convection.

Answer 2

Generally, temperatures decrease with increasing height because the atmosphere distributes itself according to gravity:  Pressure generally decreases with height because the pressure is determined by the mass of the atmosphere above some point.  At the surface, the entire mass of the atmosphere is above you bit as you go higher and higher, the amount of atmospheric mass above you decreases, so the pressure falls with height.