Here's why:
* "Splendid" is already a superlative. It means "very impressive or magnificent," and it already conveys the highest degree of the quality.
* English grammar generally uses "-est" for superlatives. You wouldn't say "goodest" or "baddest."
Instead of "splendidest," you should use:
* Splendid for the superlative (already meaning the highest degree).
* More splendid for the comparative if you need to indicate a higher degree than a single instance of "splendid."
For example:
* "The view from the mountaintop was splendid." (Superlative, meaning the view was very impressive.)
* "The sunset was more splendid than the sunrise." (Comparative, meaning the sunset was more impressive than the sunrise.)