They've been revamping old systems at the request of their patrons. It just so happens that most of the people contributing to this game are more interested in expanding/refining the serum routes than they are in the main story.
Some systems had to be changed because they were an absolute pain in the ass. If you remember the original sex system, it was a boring, repetitive time sink. I used to mash "tab > enter" over and over again grinding Dakota's corruption just so I could give her the next dose.
Making the experience leading from one chapter into another enjoyable is just as, if not more, important than the addition of a new chapter. Just because the length of the story hasn't changed doesn't mean new content isn't being added.