@justforthelulz
It also applies to manga/anime as a whole, but that's another subject entirely
Harem is difficult to write because I think the best way to write it is not only establish strong characters with realistic reasons for all falling for one guy/girl, but also give them all equal chances. Plus a harem end is usually desirable, but you have to find a way that this cast of strong-personalities can all agree with each other to share their man/woman, and not only that, but develop and progress at a pace that isn't too forced or too slow. Hence why there's a lot of shitty harem mangas that are just an excuse for the author to draw girls he would want to bang with no chance of any of them winning or no greater point to the story thematically or narratively.
That said, the harem genre is technically something that has been a thing in Japanese media since the Tale of Geniji, so it's pretty much baked into their country at this point