What i think, i dont know if its true, but in order to interrupt abilities, i.e stun, you have to be in a close proximity while this ability is casting, or it is offseted by a timer so theres a room to finish a cast os this ability, its either you have to be close and in danger of being killed, or they will have at least some time to cast. Sleep is a skillshot tho so you can miss, but people while channeling usually slow and easy target, and you can sleep people from far away from a safe position. So idk the actual reasoning, but this what came in my mind.