I think part of it is that for most asian cultures, such big declarations of love and emotion is pretty much frowned updn. that hiakru chose to leave minmay behind would have been enough of a declaration for misa...
having said that, I think even the creators of SDF Macross felt the same way.. since DYRL makes the choice much more clear and is again reinforced in flashback2012.
I think also to an extent... it wasn't that hikaru didn't love minmay.. perhaps even loved her more, but that he realized that in chosing to love minmay, he couldn't be true to himself and his calling... that Misa was a better fit for him, someone with whome he would find a better long term relationship. Minmay is Hikaru's first love... Misa is a more mature and fullfilling kind of love.