Here's why:

* Igneous rocks are formed from the cooling and solidification of magma or lava. This process effectively "resets the clock" for the radioactive decay process. When the rock crystallizes, it traps a specific amount of radioactive isotopes, allowing for accurate dating.

* Sedimentary rocks are formed from the accumulation and cementation of sediments. These rocks often contain fragments of older rocks, making their dating complex. The radioactive isotopes in sedimentary rocks may not represent the time of the rock's formation, but rather the time of formation of the older rock fragments within it.

* Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat and pressure. This process can alter the original radioactive isotopes and make dating inaccurate.

Therefore, igneous rocks are the most reliable for radioactive dating because their formation process allows for a clear starting point for the radioactive decay clock.