Why are contrails not visible from the cabin?
The distance between the engine and the start of the contrail depends on:


the engine type
the temperature of the outside air
the moisture content of the outside air. 


Usually the contrails don't form before the tail of the aircraft, but in extreme cold or humid air contrails can be seen forming just behind the wing. 

On larger aircraft you have a better change of seeing the contrail from inside the aircraft. On the 747 you can often spot them for the window of the rear passenger door.