why is there convection(and thus weather) in the troposphere but not in the stratosphere?

Answer 1

In the upper parts of the troposphere is a layer called the "tropopause". This is an area where there is usually an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height). What that means is that the tropopause is a very stable layer. Convective cloud would not be able to penetrate it, and instead often spreads out into cirrus.

Answer 2

The rise of air in the atmosphere is called convection.If an air parcel is warmer than its sorroundings,it rises above due to bouyancy and convection takes place.This convection can continue till the top of the trophosphere as the temperature falls with height in the trophosphere and the air parcel will remain always warmer than its sorroundings as it rises.
But in the stratosphere, temperature increases or remains constant with height due to the absorption of solar radiation by the ozone layer which is concentrated at these levels.Here the parcel can no longer rise as sorrounding air will be warmer than the rising air parcel and convection will stop.
Further the water vapour is confined to trophosphere only.So cloud formation never occurs in stratosphere due to the absence of convection and water vapour.