In a word it is because maaser reflects Shevet Levi's portion in the land whereas terumah is just one of the many matnos kehunah.
@CuriousYid Not fluffy at all. Bamidbar 18 says that.
First it details all the matnos kehunah (through verse 20) that the kohanim receive and that God is their portion and inheritance.
Then it explains that really Shevet Levi is entitled to land but that they were given over on behalf of klal yisroel to assist in the Mikdash and are therefore entitled to tithes which represent their rightful inheritance in the land (till verse 25).
It then closes out with that being the case the leviyim too—like all of Israel that works the land—are required to give terumah from the maaser given them as it is akin to their portion in the land. It is also permissible to eat maaser anywhere as it does not have kedusha like terumah but represents their portion in the land.
After all 24 matnos kehunah are factored in it may very well be the case that the kohanim come out ahead of the leviyim.