Why did Celestia and Luna become "Princesses"?

[Denim&Venöm 16,517]

What I mean by that based on the hearths warming tales and the three tribes, they had three forms of governance. Military Junta for the Pegasai, Representative Democracy for the Earth Ponies and Monarchy for the Unicorns. When the three tribes came together and chose the Alicorns to be their leaders, why were they made into Princesses?

Did Celestia and Luna actually belong to the house of the unicorns? Why would the other tribes agree to the unicorn system of power? They never appeared to be part of Hurricane's army, so that system of governance wouldn't be an open avenue to them. So the democracy of the earth ponies would make the most sense. The ponies of the tribes effectively elected Luna and Celestia to lead. 

So why don't we have Chancellors Celestia and Luna?  Why and how were they made princesses? Why go with princess or any of the tribal systems at all? 

[Steve Piranha 28,782]

According to the Journal of the Two Sisters, the recently united tribes of ponies came to the rationale of being governed by rulers representing the three tribes. So they went to what is now Canterlot where a tribe of alicorns lived, and chose Celestia and Luna, at the time still young fillies

[Props Valroa 26,046]

I suppose they had a King and Queen at some point who either died, or something else happened to them.

[ManaMinori 4,143]

Because Disney-ism. That's the only reason