The cell chemistry only plays an indirect role here (neither zinc nor lithium are terribly healthy in a child's body I guess).

The lithium batteries are thrown away with a fair bit of energy remaining: Internal resistance will go up too far to be useful to power something, but the cell can still electrolize when inside the body.

Zinc-air primaries on the other hand tend to be completely empty when removed from the hearing aid; they have a really nice flat discharge profile.