mentioned in the annotated screenplays. Back during development, Uncle Owen and Aunt Lars were not related to Anakin Skywalker, much less Luke - they were related to Ben Kenobi.
Discussing the second draft of Return of the Jedi:

The Emperor knew, as Ben did, that the Force is strong in the Skywalker line, and he began to search for Anakin's offspring. Ben reveals to Luke that he has a twin sister and that they were separated; Luke went to stay with Ben's brother, Owen, on Tatooine, while his sister and mother were sent to the protection of friends in a distant system. The mother died shortly thereafter, and Luke's sister was adopted by Ben's friends, the governor of Alderaan and his wife.

While this discusses the development of Jedi, it standsto reason that the idea was likely ruminating in Lucas' head back in the A New Hope days, if it was such an early idea during Jedi. Following this, Ben had multiple reasons to be on Tatooine. Being able to monitor Luke was certainly one, but it seems Tatooine was also likely intended as his original home.
From the notes, it's also a little unclear whether or not the "friends" Leia and Padme were sent to were the Governor and his wife, or if they simply adopted Leia after Padme's death.
Unfortunately, the screenplays do not provide any reason in particular why Luke was chosen to go with Ben to Tatooine over Leia. Quite honestly, it's likely just an obvious fact that most heroes in Hollywood movies, and in the