Pot radical on the left - also
罐's indexing radical (Kangxi #121).
缶 itself depicts a wide-bellied clay pot with a lid, the earliest pictograph for a storage vessel. Direct semantic match:
罐 is exactly that - a jar, can, or jug. Same radical in
缸 (vat),
罄 (empty),
罍 (large jar) - every ceramic vessel sits here.
