Two reasons. Primarily, they can't. They lack the requisite skills. Creating anything which will resonate across nations requires things they do not have and can never learn.
The second being that they just want to push an agenda and using an existing fan base pushes it the farthest fastest.
The third being an active desire to re-write anything that existed before them to comply with their ideology, so that there is no escape from their ideology.
If it's in a universe where nothing from the old universe exists, why not make a new character?
Two reasons. Primarily, they can't. They lack the requisite skills. Creating anything which will resonate across nations requires things they do not have and can never learn.
The second being that they just want to push an agenda and using an existing fan base pushes it the farthest fastest.
The third being an active desire to re-write anything that existed before them to comply with their ideology, so that there is no escape from their ideology.