Here's why:
* Delightful is a subjective adjective: It describes a feeling or experience that is pleasurable and enjoyable. The intensity of that feeling is highly personal.
* Comparative forms are used for objective comparisons: They show a difference in degree between two things. For example, "bigger" or "faster" compare size or speed.
Instead of trying to make "delightful" comparative, you can use other phrases:
* More enjoyable
* More pleasant
* More pleasing
* More charming
* More captivating
Example:
"The sunset was delightful."
Instead of "more delightful," you could say:
"The sunset was more enjoyable than the sunrise."