Because she isn't some slut whore who shows off cleavage everywhere to get attention.
Lucina is a lady and a beautiful one at that. I would court her and open doors for her and lay my trenchcoat over a puddle and help her across it by holding her hand.
All the characters are revealed with unfinished details that get fixed and adjusted down the road. Look at Palutena in Lucina's gallery, she's gotten several adjustments in the month since her reveal.
She is A cup, you can't see them though because of her armor and wool sweater
because they are obscured by armor and clothes