Crazy machines sold quite good,
and it a mix of Fun and edutainment (Physical puzzles)

On Mobile devices, things like DR Kawachima, brain Trainer etc
also sell well.

maybe the publisher you talked to simply had low quality products,
causing them to sell poor...

In the end Edutainment is partly a way to make education TV programs or similar shows, or a science book beeing an interactive medium.

Inserting rewards, unlockable features und a narration by some character do not detroy its educational idea, while
enhancing the motivation of the user.

The idea about edutainment is not to look as boring and serious as possible, but to transmit some scientific knowledge on the fly.
Even Age of Empire is edutainment is this sense,
teaching about Ancient Historical events.

"Capitalism" was also successful as game AND teaching tool in
business schools.