Question

Why only Ash's Greninja could Battle Bond, and not any other Greninja with its trainer (like Ippei and his Greninja)?

Comments

Two words: Fan service.

Answer 1

Because Ash is the main character.
They have to make ASH have this kind of thing, because he is the main character. Also could be because Ash has a better bond with his greninja.

Answer 2

Pk was correct, yes but I also want to add that ash's greninja was noted to have a special ability. It was noted in one of the episodes that it was special, and others were not able to do the same. The reason froakie ran away from the other trainers, because it knew they were not right for it and weren't able to let it do that. While others may have had a strong bond too, they didn't have that special ability that ash's had.