Because most of these games are money making scams.
That is very sweeping without any arguments...
They are mainly people trying to make quick easy $$.
Even if you make $2000 a month, that doesn't cover the average VN's effort at all, considering all the time spent on story, rendering, animation, renpy configuration. It would be an hourly wage of under $1. Believe me or search different forums, you will understand what I mean.
90% of devs do it out of passion, the financial support is the recognition, honor, gratitude and motivation that their content is valued by peers. Become a dev, go to internal forums and you will find out for yourself.
90% of members here on F95 are ready to pay $2-5 for a crappy coffee. For 15 minutes of enjoyment. However, exactly this 90% are not ready to give even $1 to a dev from which they consume a VN, jerking off several times or several hours. And that with every new release. This is the "mindset" of the internet-generation. There is an unwritten law like "porn is free", but not coffee, food, etc. And all devs have to live with that...
There is no "quick easy $$" if you make 3dx content nowadays...
Then you have some devs who throw out idea's and if they dont get "enough" support they toss that game aside and try something else until they get a money maker.
I am sure that in some cases it is. But that's not only due to lack of revenue, but generally due to the fact that too few people like what you do. It affects the motivation to invest an extreme amount of time in a project that most people think sucks or completely igorize.
Another reason can be the fact that you develop as a dev very quickly, create better characters, better environments, think up a better story and thus grow beyond the level of the previous VN. Your previous VN loses its appeal for you because you don't develop with it and/or can't make drastic changes in the "old" VN.
I´m glad I still have ideas and passoin for Isolated Pleasure, because I see a lot potential. But I got a little tired and need some fresh air. Furthermore, I´m still collecting and preparing all necessary assets for the plot, the environment and story. My plot is not "standard"... it´s not a fucking ordinary house in the west with 5 rooms... I need to render a post-apokapyptic world.