Robots are expensive and not very self-sufficient. (They can't usually fix themselves or build more robots using resources from the environment.) For now, getting a Mars base going is hard but it's probably a more attainable goal with current technology.

Mars is also a more hospitable place; it has gravity, atmosphere, water, etc.. It's possible to create methane fuel on Mars.

There are also more reasons for people to want to visit Mars that aren't motivated by becoming a multi-planetary species. For instance: science, tourism, entertainment (sports, movies, reality TV, whatever), and low-gravity benefits for people with mobility issues who would be stuck in a wheelchair in Earth gravity.

That isn't to say we can't or shouldn't go to Ceres as well.