The earthen vessel - Unglazed pottery would absorb some of the juices of the meat: and a vessel made holy could not be put to any other purpose.
Related Commentaries of Leviticus 6:28
Leviticus 6:28
28 But the earthen vessel wherein it is sodden shall be broken: and if it be sodden in a brasen pot, it shall be both scoured, and rinsed in water.