Strange English: Crispy English

You can use "crisp" to describe many things. Food, weather, paper, people's personalities...

• I like to eat my cereal quickly, while it's still crisp. If it sits in milk too long, it gets soggy.
  
• It's finally fall! The air is crisp and cold.
• You can tell this book has been well-loved. Its pages are soft and curled, no longer crisp.
• Her attitude was crisp when I told her I was coming in late.
  

But you can only use "crispy" for foods--maybe only a few foods.

• Potato chips, lettuce, deep-fried foods, and maybe apples can be crispy.
  

Why? I have no idea. But if you call anything else other than food "crispy," it sounds totally weird.