Because Mercury DID have plate tectonics. Mercury is just no longer tectonically active,
"...Each planet has a unique history and unique tectonic features. Large planets, such as Venus, Earth, and Mars, are large enough to have remained hot inside and still have active tectonism. Smaller bodies, such as the Moon and Mercury, have cooled further and are not thought to be presently active, but their features tell geologists of an active past. ..."
