Over the years, there have been multiple anime that have done exceptionally well globally despite being not as popular in Japan, and Solo Leveling may just be the latest addition to this list. Anime and manga have begun to acquire an even more diversified fan base in recent years. This shift is reflected in the various languages dubs and translations are readily available in as of late and the overall increased accessibility of anime and manga.
it did well abroad because the manhwa adaptation had standout visuals, at least for the time, to the point that it tricked you into thinking it wasn't a generic power fantasy. that's pretty much it as far as I can recall of 16 year old me who was actually invested.