Revealing mistake: In several of the sanatorium scenes, you can see large pieces of cardboard attached to the lamps to affect the lighting.
Revealing mistake: When Renfield walks up to the front door of the castle, he is in a court yard where the only light comes from torches. But the light is enough to make him throw a clear shadow on the wall, and the shadow never flickers as it would in torch light.
Answer: So he could seduce Mina slowly with his hypnotic eyes, Dr. Steward would have been in the way.