Why do they make so much money? ... I think this is a nice litte game (all that translation during dvelopment is a bit over the top I think).
Development seems to be quite slow for that budget.
Theres some obvious reasons for success.
Right protagonist, a meek black haired girl. You might see this archetype in other quality media with same focus.
Right themes, mix of voluntary/forced corruption justified with a goal in mind that requires money.
The devs know how to construct the game, they understand the basics and the engines limitations and work with them.
Theres no obvious wrong design decisions and the game is visually appealing 2dcg.
Unique design choices like being able to switch to the guys pov and dream sequences.
Some minor stuff like multiple languages supported, game guides you ingame + a walkthrough exist.
Generally good marketing, 4 different platforms supported and so on.
How many games tick ALL those boxes? Not many.
Now as for speed of development, money doesn't magically turn into content.