Why is Greninja Ash's only bonded Pokémon? If Ash bonded with his Greninja that he had for one generation, but he won't use it anymore, why haven't Pokémon like Pikachu or Tauros bond with Ash instead?
In the same way that not every pokemon has a Mega Evolution the only pokemon known to have the bond ability so far is the Greninja.

True that by now we can believe that Pikachu and Ash have maximum trust, one of the requirements for the Bond phenomena to happen.

But that might not be the only requirement, or Pikachu species does not have this bond ability.

It remains to be shown further if other pokemon than Greninja have that ability or not.