In Stratosphere, the temperature increases proportionally with an increase in the altitude. Here, the air flow is less turbulent which is quite optimum for the jet planes to fly. Even the wind speed is almost gradual and constant as the air flows in parallel fashion with the earth rather than up and down. So, as constant balance of all the factors lead to the best sphere for the jet planes to fly properly with correct supervision and vigilance
Stratosphere lies above troposphere .In stratosphere , temperature increases with increase in altitude .This implies that at lower part of stratosphere the temperature remains constant(up to 20 km) and then increases.
Jet Planes fly in lower  part of stratosphere because this layer is free from water vapour,clouds ,dust particles, thunder and lightning. This layer has strong, steady and horizontal winds[from west to east) which offers less turbulence to jet planes.