Here's why:

* Igneous rocks form from the cooling and solidification of magma (molten rock) or lava. The Earth's mantle is largely composed of magma, and this material rises to the surface at divergent plate boundaries (where plates move apart) to create new oceanic crust. This crust is primarily made of basalt, an igneous rock.

* Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are subjected to intense heat and pressure. These conditions occur deep within the Earth's crust and are also present at convergent plate boundaries (where plates collide). The immense pressure and heat transform the original rocks into metamorphic rocks like marble or schist.

While sedimentary rocks are also found on Earth's surface, they are not the primary constituents of tectonic plates. Sedimentary rocks form from the accumulation and cementation of sediments, which are often derived from the weathering and erosion of existing rocks. They primarily reside on the surface and are often deposited in oceans and lakes.

Therefore, igneous and metamorphic rocks form the bulk of tectonic plates as they are directly involved in the processes of plate formation and movement.