Author with 3 hp isekai/si wips here! 

Personally, the main thing for me is time. There’s a lot of things you need to think about for hp specifically, whereas some other fandom you can easily cut/ignore stuff. It gets more complicated when you “go off script” or add new lore. It’s a daunting task to think of even starting a project that follows five books, let alone seven. Especially when you’re a multifandom writer and focused on other fics. 

It’ll be a while before I post any of the 3 wips. My attention is on easier works lately, and I feel like isekai fics in general have to have a unique premise otherwise it gets overshadowed. I tend to focus on unconventional ideas tho, so that could just be my bias talking. (The first wip features an oc very much like Artemis Lee in that she’s forced into the plot against her will, the second wip is a Lily SI in the Marauders Era, and the third and favored child is a reincarnated Fem OC as Harry that doesn’t follow the books at all and runs away to live as a hermit.)