Mate in 4

Why not 1. Nxh6 ?
Nxh6 doesn't threaten mate on the next move (Nf6 does), so black isn't forced to capture the knight.
Why not 1. Nxh6 ?
Nxh6 doesn't threaten mate on the next move (Nf6 does), so black isn't forced to capture the knight.