Here's why:

* Strata are layers of rock, often formed by the deposition of sediment over long periods.

* Sedimentary rocks are formed from the accumulation and cementation of sediments like sand, mud, and organic matter.

* Igneous rocks are formed from the cooling and solidification of magma or lava. They typically have a more chaotic, crystalline structure and don't form the distinct layers seen in strata.

While strata are primarily associated with sedimentary rocks, there can be some exceptions:

* Volcanic Strata: Some volcanic eruptions can deposit layers of ash and lava, which can form distinct strata. However, these are still considered sedimentary deposits.

* Metamorphic Strata: If sedimentary rocks containing strata are subjected to intense heat and pressure (metamorphism), the strata might remain visible in the resulting metamorphic rock.

In summary, strata are most commonly found in sedimentary rocks due to the way these rocks are formed through layering processes.