Exactly why is unknown. All the beasts was shown in the second art book with his comments. For the slug he said:
- "The Six-Tails has a dripping feel to it¹. However, it doesn't spit lightning or anything (laugh). Utakata [the jinchūriki] is a charming character who uses soap bubble (bubble-style) ninjutsu."
¹ Like falling rain.
And for the beetle he said:
- "The concept for the Seven-Tails was an insect. The armour has the feel of an insect's exoskeleton. I made the tails into wings. The jinchūriki is a girl, for now. I haven't completely decided yet (laugh)."