Can you explain why it's easier?
It's easier because Solvalley is a collection of linear stories. I just need to make the images/animations, write and then with a single line of code (literally), this scene is already implemented in the game.
In the case of HS Tutor, just to give an example that I'm working on now, the player can visit Claire at night in her room where there is a hot event. After this event, the player can find Claire in several different places. It can be at school in the morning, at school in the afternoon, at the park on weekends or at her house at night. For the game to make sense, I need to take all of this into consideration. The player can not find Claire in the park after this event, and she act as if nothing had happened. Before and after each event, I need to make sure everything is working properly and that the new lines are in the right places.
The difference, then, is that while in Solvalley girls are "buttons" that, when pressed, make their storylines run, in HS Tutor girls have their own existence regardless of where the player is on the plot. And keeping the girls' reaction consistent with what's going on requires a lot of hard work and attention.
The problem is that this is kinda hard to program alone. Even semi-professional games often completely ignore these things or simply neglect it by assuming the player will not care, as the Summertime Saga does, which is often extremely incoherent (when I played the game, I even got to see some girls comment on things before they happen on the plot lol).