Why can't I use King of the Swamp for Crimson Nova Trinity the Dark Cubic Lord's fusion summon?

Must be Fusion Summoned with the above Fusion Materials and cannot be Special Summoned by other ways.

It's literally the first line of text on the card.

Must be Fusion Summoned with the above Fusion Materials and cannot be Special Summoned by other ways.

This means that the materials used must have the name "Crimson Nova the Dark Cubic Lord". You may not use a fusion substitute for it. Of note, it doesn't need to be the cards original name, so a {{Elemental HERO Prisma}} that changed it's name to Nova may be used as a fusion material.