Because the Viltrum empire is vast with many planets. Ruling over such a vast territory with a tiny population is naturally going to be difficult. Some of those planets are likely pretty unhappy about being conquered and certain elements would rebel if the viltrumites guarding them left their post to go after Earth.

Taking soldiers away from their posts means fewer guards for important trade routes/less people to maintain control over planets/less people to fight important wars. And every viltrumite is needed because of how low their population is.

Additionally, it’s not like every viltrumite is capable of solo conquering a planet. That duty is explicitly given to the very best viltrumite soldiers. The average viltrumite soldier might simply be too weak to overpower a world like earth. Even Omniman nearly died against the Guardians. A typical viltrumite soldier would probably have died.

As for why Mark was given the responsibility, probably just to fuck with him. Making him fix his father’s “mistake” knowing it would be hard for him. And to prove himself worthy of living. So it was basically just a way of punishing Mark and breaking him in to the viltrumite way of life.