To give you an exact reason is kinda difficult as it is not one single thing, but rather a couple of things. I guess NTR could be large factor especially how in your face it is or at least it felt that way to me, but I would not say it is the sole reason. In addition to the NTR theme I felt that more attention and care was put into the character of Mel based on what scenes and dialogue is/was available. Basically from my view point Mel received a lot more of what I considered interesting and unique content and comparing that to what the player could actually do or influence I feel/felt cheated. It feels/felt more of a Mel world than anything else especially, in my opinion, how pervasive he is through out the beginning portions of the game. Then there is the foreshadowing of the neighbors which I mentioned above which again I felt just added to the issue.
From when I first played the game very early on, I really liked the theme and the art style, and as it went along the moment he added the pregnancy sex animation I felt that this was a great effort to add details to the game, as I had not seen other games do that at the time, I quickly started supporting rather than just downloading the game (it was the first game I gave money to, and the one that lead me to look for other games XD).
I was a $15 supporter for a very long time, and I was happy, but same with you, I felt that it was getting many updates and effort towards Mel, with some of the artwork he posted on patreon involved Mel with Cas and Olv, it slowly kept turning me off the game (These art works may not be "lore" but I still din't like em much, great artwork, but still wish it was MC and not another character). And now the way the story is going (other people having books/powers), it doesn't seem to be the game I was expecting where you the MC goes on a corruption rampage and get what ever you want, or succumbing to the succubus you released from the book.
I do not mind that Mel is in a relationship with Cas, or the possible neighbors having a relationship in the next updates, what I want is to be able to change that at my will, and it seems more effort is given away from that, or at least, will be many updates away until this is possible. I stopped supporting and keep an eye on the change log to see if some off the new updates peek my interest, but for a while, so far none have.