Kassandra is the protag of the book adaptation, Alexios is the face of the games artwork and promotional material. Chose Kassandra cuz she's the badass, got better voice acting, fits the role better, has the better capturing and Alexios is a whiny wiener, stupid af, thinks he's important for anything or to anyone while actually being played like a fiddle and behaving completely bananas. Pretty sure I read somewhere that the devs consider her to be the canon protag as well. Alexios seems to be more for marketing and just an option, however Kassandra is confirmed to be the character in the adapted novel.
I see.. Doesnt mean it couldnt be for balancing reasons though.