Does the law of crosscutting relationships involve sedimentary rock only? Why or why not?
No, it does not only involve sedimentary rocks.
Why? It's because igneous rocks can also cut through, as a result of a pluton intruding through the preexisting rock.
It is simply a relationship between two features - not exclusive to sedimentary rocks - that aids researchers in determining the relative dates of both features.
The "cutting" can be represented by a volcanic dike, for example, which is composed of igneous rock. In this case, the dike would be the younger of the two rock types, therefore establishing a relative age of each rock (this means that I can reasonably say that the igneous rock is relatively younger than its neighboring strata).
Take a look at the picture below for an example of an extrusive igneous rock cutting through a relatively large and older sedimentary outcrop.