Here's why:
* It describes a noun: "The future is doubtful" (describes the future). "He was doubtful about her story" (describes him).
* It doesn't act like a verb: You can't say "He doubtfuls her" or "The future doubtfuls".
However, "doubt" can be a verb or a noun.
* Verb: "I doubt her honesty."
* Noun: "My doubt grew stronger with each passing day."