Energy economics. We have to grow food. Robots build fusion power plants. Available ecological niches are greater for robots than for humans.
They don't have to be hostile. Let's assume robots are model citizens. If they're as smart and creative as humans, we become a minority, and eventually, insignificant.
I don't know about you, but when I boldly go where no man has gone before, I don't want to find robots have beaten me there and already set up a McDonald's and Starbucks.