There is no definite answer, but the most likely reason is because Torterra has the least base stat total of all Sinnoh starters. Torterra's BST is 525, Infernape's is 534, and Empoleon's is 530.
This is also shown in other scenarios. Venasaur also has a base stat total 525, and similarly to Grotle, Ivysaur evolves at level 32. I suppose GameFreak is deciding to making the weaker starters evolve earlier.
Source: Knowledge and the PokemonDB Pokedex.