Because it's less dense than the mantle rock.
The density of mantle material is greater than that of crustal material. Also tectonic plates actually include a little upper portion of the mantle which rides on top of a, for lack of a better term, "Juicy" layer of the mantle known as the asthenosphere which is about 1% liquid allowing for "flow" of the plates over top.
crust is lighter in density than mantle , like oil is lighter than water thats why it floats on water, when earth formed all lighter elements came up and made crust
can u see my question?? just one look??