Here's why:

* Unconformities represent gaps in the geological record. They indicate a period of erosion or non-deposition, where older rock layers were removed or simply not formed.

* Sedimentary rocks form in layers. These layers are laid down over time, and an unconformity marks a break in this continuous deposition.

* Igneous and metamorphic rocks form under different conditions (from magma or heat and pressure) and don't typically form the layered structures necessary for an unconformity to develop.

However, it's important to note:

* Igneous intrusions (when magma pushes through existing rock) can create a contact between igneous and sedimentary rock, which could be considered an unconformity.

* Metamorphic rocks can sometimes be affected by unconformities. If sedimentary rocks were metamorphosed and then eroded, an unconformity could be present between the metamorphic rock and any younger sedimentary layers.

So while unconformities are most commonly associated with sedimentary rocks, they can occur in other geological situations as well.