The MC and Alex have known each other for a long time by now, but they have only known Isa for a very short time. So what I think BTT is doing right now for the rest of the story to make any sense, is to build the relationships with Isa. I think BTT has stated that all three of them will always be friends, regardless of which path we choose to play, but for Alex and Isa to develop that friendship they need to spend time together, so that's what we're seeing now. The MC and Isa have spent more time together, so the focus has shifted a little to Alex and Isa. All the characters are starting to like each other as friends, laying the foundation for the paths where Isa can be romanced.
Pacing is always difficult, what happens over a few weeks in game-time can take a year or more for a developer to storyboard, script, code and render. And to make the in-game time feel realistic and not rushed, while keeping dev times manageable there are always trade-offs on both.
We can absolutely have different opinions on those trade-offs, but all I can say is that they work for me and that I appreciate the slow burn in this game. I'm really enjoying watching these friendships develop.