Here's why:
* Clever is an adjective, and "more clever" is the comparative form.
* Soft is an adjective, and "softer" is the comparative form. However, "softest" is the superlative.
* Worse is the comparative form of the adjective "bad".
* Incredible is an adjective, and "more incredible" is the comparative form.
Let me know if you have any other word puzzles!