Starly is like the only Pokemon in Sinnoh Ash made an effort to capture. All the others practically begged to be caught by him.
That's because that adds character development for the Pokémon's part. They have to have some sort of motive to be caught by Ash.
However, it could be argued that Buizel made it very hard for them to get him to join the group.
As for the episode itself, I find it kinda strange how there was only a single female Hippopotas in the herd. Oh well.