Here's why and some alternatives:
* Fabulous is already an adjective, not an adverb. Adverbs modify verbs, adjectives, or other adverbs.
* "Fabulous" has a unique flair that's hard to replicate with a simple "-ly" word. It implies both amazing and stylish.
Alternatives:
* Wonderfully - Good if you want to emphasize amazement.
* Splendidly - Suggests grandeur and magnificence.
* Exquisitely - Focuses on fine detail and elegance.
* Remarkably - If you want to highlight something unexpected.
Phrases:
* In a fabulous manner
* With fabulousness
* Fabulously (although this is technically an adverb, it's less commonly used and sounds slightly forced)
Context matters: The best way to describe "fabulous" depends on what you're trying to convey.