Probably the title? "Slime Girl" is pretty simple, yet it's something that people would randomly type in the search bar. Also, it got an interesting premise that comes down in the early chapters. Yet back then I didn't even have a proper prologue nor was my writing all that great (so that may not have been it). I edited a lot in the first chapters over the years.
I think foremost it was a great degree of luck. I got into first place trending, people saw, more went into it, which triggered I get another week into trending, more exposure, repeat, and so on. I guess I also had a good starting amount of chapters to get people engaged. So it mattered when the read through.
Also, I'm very sorry, but I can't completely remember the exact number over the corresponding weeks back then. I'm halfway sure it was over 200 after three and threehundred after five. Yet I can't say if it was threehundred after two.