Why did God promise to bring Caleb into the land he had spied out?

Answer: He had a different spirit

God said, "My servant Caleb has a different spirit and has followed Me wholeheartedly" (KJV: "another spirit... followed me fully").

“But because My servant Caleb has a different spirit and has followed Me wholeheartedly, I will bring him into the land he has entered, and his descendants will inherit it.”— Numbers 14:24 (BSB)