She strikes me as having a kind of self-confidence in her role as a wife that the other two did not have. Ivana became competitive and grasping. Marla — well, when you marry someone you cheat with, things are broken from the start. He really seems complemented by Melania.
It comes down to character and personality.
Melania fits Trump in a way Ivana and Marla didn’t.
The woman isn’t a feminist and prefers not to be the co-wife.
I think that’s the way The Donald likes it.