The customer base didnt out grow the characters. People in there 30 still love nintendo, i cant say the same for disney. So adults are willing to buy amiibos to collect the figures.
I'm pretty sure that there are people in their 30's that love Star Wars and Marvel superheroes.....not to mention some of the classic cartoons.
That stuff arguably has broader appeal than Amiibos.
I have no idea why Disney couldn't make this work -- it seems like they just lost interest.