reducing IAS means pitching up, which means the component of aircraft weight opposite to the flight path increases, thus reducing the excess power available.
Also your L/D deteriorates.
Think about pointing at the sky. Then your going backwards (unless you're in an F-16).
Or a helicopter.
Helicopters often have nonzero climb rate at zero TAS, and therefore zero Vx. But they commonly have climb rate at some nonzero TAS that exceeds climb rate at zero TAS, and thus they possess nonzero Vy. They also have higher ceilings at nonzero TAS than at zero TAS - and therefore for a certain height range possess nonzero Vx in addition to a nonzero Vy.