Moshe sent twelve spies, one for each tribe, and chose a leader of that tribe. His intent was for the spies to explore the entire land of Eretz Yisroel and to see that there was a portion appropriate for each tribe.
Therefore, he sent a leader of the tribe, an individual who knew the needs of his tribe, and could tell them upon his return that there was a portion of Eretz Yisroel that was appropriate for them. Although the land had not yet been divided into tribal portions, however there was a portion of land that was destined ultimately to belong to each of the spies within the portion to be given to his tribe. Because they did not survive long enough to actually inherit the land Yehoshu and Koleiv inherited their land on their behalf so as to be able to elevate and refine the land as they were supposed to.
