1. He's the main character,
2. Minato specialized in unique sealing techniques, and it's a result of that,
3. Their bond is still incomplete,
4. The other Bijuu take over their hosts completely during transformation, Kurama never fully takes over Naruto,
5. When Kurama was about to break the seal, he originally assumed his normal form,
6. Some sort of plot mechanic foreshadowing something else,
7. The Leaf Village is just that much better,
8. Kurama is the most powerful Bijuu,
9. I'm sure I forgot something, so..some other reason.