To most intelligent races, Earth was an obscure, primitive little planet in the boondocks, with little to recommend it to traders, or the intergalactic scientific community. Besides, according to the laws laid down by the Shadow Proclamation, it was a proscribed planet; going there for any reason could land you in trouble so most folks kept well away.
Naturally, there were exceptions, alien outlaws and warlike races intent on conquest. Not everyone could be trusted to uphold the law.
Which was where the Doctor came in. Someone had to protect the human race until they could take care of themselves!