I don't know where you're from but in Amerikkka a white man with money is tier 1 for women. Miz could definitely get badder looking women than Maryse, but listening to him gush over her in interviews you can tell that blonde white women are his only preference.
I think it's more that they got together when he was a midcard guy who people didn't like very much, and before he really got his promos together like he has. He was punching above his weight then, so there's always a lot of devotion when you get someone you thought was beyond you.
She's on baby 2 and has fallen off a good deal, whereas if they ended Miz could pretty much go down to NXT and probably have him a Tegan, Xia or a Taynara within the week. Definitely could go get some IG girl. But... dude loves his family, and that's cool too.