Why is the lithosphere rigid and the asthenosphere plastic, even though they are both part of the mantle?
It boils down to temperature: the asthenosphere is warm enough to deform plastically, while the lithosphere is colder and rigid.
You can pull warm taffy easily, whereas cold taffy is hard to pull and likely to break if you try. So it is with the rock in the upper mantle, except that "warm" in this case is about 1300°C or more. The boundary between the lithosphere and the asthenosphere is conventionally the isotherm in the mantle at that temperature (https://en.wikipedia.org/wiki/Asthenosphere).