Here's why:
* Majestic Star Dragon's summoning requirements: Majestic Star Dragon is a Fusion Monster that requires "Stardust Dragon" and a monster with "Star" in its name as Fusion Materials.
* Stardust Dragon Assault Mode's effect: Stardust Dragon Assault Mode's effect doesn't involve Fusion Summoning. It's a Special Summon effect that allows it to be Special Summoned from your hand if you control "Stardust Dragon".
In essence: While Stardust Dragon Assault Mode is a powerful card and can be used to support Stardust Dragon strategies, it cannot be used as a Fusion Material for Majestic Star Dragon.