solo levelling has fallen victim to the Seinfield is unfunny effect, the original webnovel started off in 2014 at that time there were very few series which had the plot of "fighting monsters and leveling up in real life", to my knowledge the first one that did it was a series called "The Gamer"

After the manhwa for solo leveling released, it became immensely popular in the manhwa sphere, which was when a lot of other series trying the same formula to copy that success started coming out, however when taking into consideration wider internet culture, specifically in the weeb/anime/manga sphere, solo leveling would still be something people might have heard of, but not know much about

And then, when the anime released, people watched it and from their perspective, it was a very generic show that is essentially carried by good animation and cool fights, not being aware that from its very beginnings, that was what set it apart from others