Actually, Ash has caught more Pokemon in a shorter amount of time than ever before with the Best Wishes series.
As for Ash catching all the Grass type starters, Bulbasaur and Chikorita were just given because they are starter Pokemon, not because they are of the Grass type. He caught all the starters in Gens I, II, and V. While the reason why Treecko was chosen to be given to Ash over the other starters cannot be said for sure, the most likely explanation is that it is because it is more human-like than the other Hoenn starters, which leads to a better character development, important for a primary Pokemon on the main character's team.