Why has this game become so popular? The biggest factor is the fact that there are many female characters and harems. It doesn't have a story to tell. Also, there is no music in the game, no sound... For example, when the last episode took the field in the last scene, very good ambiance and ambient sounds could be made...
As one of the original posters in the thread way back when, it first started taking off due to weekly/bi-weekly updates. BD consistently released small updates for a while too. At one point, this was probably the most hyped new game around. Reaching a few thousand patrons very quickly.
Another factor is, yes, this is basically the ultimate harem game. Or was. Tons of girls, solid/good renders and it had interesting content too, with the impregnating stuff and trans content. Plus BD himself was active and friendly.
Friendly dev + popular genre + frequent updates will lead to popularity.
Then issues started to happen. BD has always shared a lot about his personal life, but it got to a point where he was vastly oversharing. I think the issues were legit, at least early on. But it opened himself up to be critiques pretty strongly. At a point, people aren't going to believe you anymore.
Then he started to miss updates and have lots of delays due to those personal issues. Imposter syndrome, personal issues, mental issues etc. This has continued to this day. Which has caused him to be lumped into the ISCTOR category of developers, fair or not, because people either don't believe him anymore or think he is trying to manipulate/guilt trip his fans to keep supporting him.