Why isn't Cadance and Shining Armor considered King & Queen after marriage?

[Rissian]

If you re-watch the season 3 premier Twilight says the crystal empire was originally ruled by a queen before Sombra took control of the empire. It's right after she finds the book.

"A crystal fair according to this book, it was established by their first queen"

Just something interesting about what I found out after re-watching the episode and I thought it might add further discussion.

It's interesting because it shows that princess is indeed not the highest rank in MLP's world. And the fact that she wasn't an evil queen, as before Sombra the crystal empire was full of love and happiness since that is what powers the crystal heart.

[Prosterguy]

reason is, They still have no children. An Empire ruler will only be declared as king, or queen when they have children and when the king and queen dies, the children has the right to be a king or a queen (even if they're not married or have kids) but still, it depends on if the empire has a ruler once before. so basically Caedance and Shining's Crystal Empire is a new empire. 

[AaronMk]

A better question is why Cadence isn't Empress and Shining isn't Emperor-["Lowborn pleb"]-Consort.

[20firebird]

I was under the impression that, in Equestria, 'princess' is joint and 'queen' is singular. The Crystal Empire is still technically part of the Equestria Monarchy, so Cadance is a princess. No idea what's happening with Shining Armor.