Original: Many of the thematic and stylistic differences arose because Tolkien wrote The Hobbit as a story for children, and The Lord of the Rings for the same audience, who had subsequently grown up since its publication. Some differences are in minor details; for example, goblins are more often referred to as Orcs in The Lord of the Rings.
Simplified: Only later, when Tolkien wrote The Lord of the Rings, did he integrate the story of both Hobbit and The Lord of the Rings into his Middle-earth world.