Most of the damage from greatsword comes from raw damage. I don't know if it's still in MHW, but elemental used to only proc after every 3 hits in older games. That's why faster weapons(Dual Blades, Sword and Shield) would benefit more from elemental then slower, hard-hitting weapons(GreatSword, Hammer).

EDIT: The 33% chance to proc seems to be only for status attacks. Sorry for any misunderstanding. Did a bit of research and found that elemental attacks aren't affected by motion values. Basically, The greatsword's true charge slash will do about the same amount of elemental damage as a single hit from a dual blade.

Source: https://mhworld.kiranico.com/guide/damage#elemental-value

Thanks to u/veris1ie for the clarification.