I hope I don't end up writing an essay here but here goes why, at least for me.
I like worldbuilding.
Japanese LNs and manga, usually, lack that. This story has a lot of lore, and much of it is either explained through the eyes of the MC while he's growing up, some other bits of lore we learn as we go, and of course, sometimes we get heavy exposition dialogs. Point is, we know how their world works, and it feels cohesive.
We know how the magic system works, how they cast a spell (we even get a step by step guide - instead of random magic circle that just works, or an entity inside MC's body therefore -> MC can shoot lasers ), we know about different races and their language (most stories don't even bother with this, everyone understands each other's language, regardless of where they go or how far they travel), their politics, and more.
I also like characters that have to put some effort, instead of those that immediately get everything handed over to them.
The MC is mentally messed up. He was too fat to fit in, and too ugly. Got himself bullied and traumatized. At age 30+ he was a broken, sexually frustrated mess of a virgin, with extreme anxiety, literally to the point of no return.
He became a HikiNEET (like a Japanese incel) and developed a twisted personality as a result. Even though he has struggled for decades with his mental demons, he became complacent licking his own wounds and leeching of his relatives, instead of putting an effort to change and better himself (the dude was fapping to visual novels while everyone else was mourning his relatives' death ffs).
On top of that, the dude was too broken and too arrogant to both seek nor accept help, and yet... despite all this, when he's finally outside after decades of being a disgusting hermit, he died trying to save someone.
Personally, I can't relate too much to him (despite my username lol) but I do get the bullying part, and the becoming complacent part, and the being lazy instead of putting real hard effort into achieving something. It's way easier to blame the way you look, or the money you have, or society for your suffering. Changing and bettering yourself takes time, effort, and sometimes it doesn't even yield the results you want or expect. His story isn't something too unrealistic, actually, I think that it resonates even more nowadays than it did when the webnovel was written in 2012.
Either way, my point is, if you ever felt regret about something you did, or maybe if you said this to yourself: wish I could start over/wish I could go back in time and try again, you will like the story, because it feels rewarding. It feels rewarding because you get to see someone (that you would probably dislike and dismiss in real life) at the lowest point of his life, slowly changing, recovering, trying, and putting genuine effort into living life to the fullest.
If you can't relate nor care for any of that then, the story isn't for you. If you're here for the fights, there aren't that many. And if you're here for an overpowered MC, this isn't the right MC for that either.
TL;DR
MC is a flawed human, he feels like 4ch user that you can learn to like. Also this is a complex and cohesive world, instead of the (ironically and most likely directly plagiarized from this very LN) cookie cutter: big titty goddess got you isekai'ed! You're now perfect in every way and live in generic Europe! Here have level 999 gear, go get a harem, and conquer this bland world without any effort.
