Why Demeter called the goddess of harvest?

Answer 1

All of the Greek gods and goddesses aspects are derived by their names. Demeter comes from PIE (proto-Indo-European: the source for most of the languages in Europe). IN PIE, "-meter" is a form of "mater" which is mother. "De-" comes from either a diminutive that means earth or grain. Either way, it seems clear that Demeter was one of the many personifications of "mother earth"

Answer 2

Demeter is called the goddess of the harvest because she is responsible for agriculture, fertility, and the cycle of the growing season. She is associated with the bountiful harvest and ensuring the growth of crops to sustain communities. Demeter's importance in the agricultural cycle led to her being recognized as the goddess of harvest in ancient Greek mythology.