Funny answer - because beatles weren't that good to warrant a sequel.
Real answer- because not in a single side route, on my memory, they address any of the issues that started in those relationships. From the quite obvious cases like sexual assault just getting brushed away, or the whole multiple mental manipulations by Chiaki, or Koharu existing.
Side routes are an addendum to the main 3 characters, which is fine for symbolism, but also leaves you wondering if Maruto decided that the resolutions enough to justify relationships which are built on extremely questionable grounds (suspension of disbelief, I know).
Plus the whole Kazusa's true ending ends up with couple not living down their "betrayal" which is presented almost like a form of PTSD, to the point it becomes so god damn humorous that even in the "happy" ending they are not happy at all, apparently, since they have this little devil sitting on their shoulder everytime they have sex or something. Obviously it is open ended and is hinted towards the idea that they can be friends, but it's just funny that the entire relationship is 1 step away from mental breakdown like that, filled with depression, regrets and outright obsession towards the central figure in the story.