Here's why:
* Greek number prefixes are typically used for multiples of ten (e.g., deca-, hecto-, kilo-), or for specific numbers like mono (one), di (two), tri (three), tetra (four), penta (five), hepta (seven), octa (eight), ennea (nine), deca (ten).
* Hexa- (from the Greek "hex") is used for six, but it is primarily associated with things that have six parts or sides (e.g., hexagon, hexameter).
Let me know if you have other questions about Greek prefixes or numbers!