because the writing team does the vast majority of the work on social links, there isn't much else to spend there time on as the story will be mostly finished well before the development on the game.
If you still are expecting social links then how much time are attention are you actually think is being diverted to turn some of them romances. And in persona 3 vanilla you cant just do social links without romance unless you just ignore the romanceable characters
Except for... you know... creating and programming all the animations, setting up all the dialogue trees, dialogue flags, multiple routes, and making sure that there's nothing like infinite loops or game breaking bugs as a result of a player's choice.
Also, writing teams do not usually work independently of the rest of the team like that. =P