It seems like Kakegurui is finally starting to get the recognition it deserves. With celebrities like Megan Thee Stallion and Poppy cosplaying as the main character Yumeko and being open about their love for the series, it's opened the floodgates for new fans to discover this high-stakes anime. It also helps that a global streaming service Netflix has both seasons available. But, seeing as the manga was first published in 2014 with anime adaptation following in 2017, why is it just now becoming popular?
Kakegurui follows transfer student Yumeko Jabami as she takes part in extreme gambling as part of the high school's strange hierarchy. The students all gamble their way up said hierarchy, being the children of some of Japan's wealthiest families. Should a student fall into debt, they become a "pet" to the other students, being dubbed "Doggy" or "Kitty" depending on that student's gender. This debt has the chance of following them after graduation should they not be able to clear it before then. However, unlike most students who gamble in order to gain popularity and connections, Yumeko gambles for the thrill of it. She's a compulsive gambler (hence the title of the anime) that craves the feeling that the activity brings -- taking part in increasingly dangerous games in order to satiate that craving.
Part of its late-stage popularity outside of Japan could have to do with the accessibility of the series. Many anime fans use Crunchyroll or Funimation to stream anime, but Kakegurui isn't available on either because Netflix holds the streaming license. The benefit of using specialist platforms to watch anime is that you can watch some series for free, but with the caveat that there's usually a bunch of ads and you have to wait a week to watch the newest episode. This is great for younger audiences who don't have access to other streaming services like Netflix or can't afford to pay for such services.
However, if a show isn't available on a site like Crunchyroll, then unless they go to some shady, illegal website that may be riddled with viruses, they aren't going to be able to watch it. There's always the manga, but not everyone goes out of their way to read the manga, especially if they have busy lives. It's much easier to stream an episode or two while cooking dinner than it is to go to the store and pick up a copy while trying to find the time to read it.
Another reason could be because gambling anime isn't a popular genre with foreign fans. Many of the most popular series fall into the isekai, shonen, battle, or mahou shoujo (magical girl) genres. Gambling anime is an unusual concept, so unless someone goes out of their way to watch it, many more casual anime fans probably don't even know it exists. Though not technically considered gambling, pachinko parlors being as abundant as gas stations in Japan might contribute to Kakegurui's appeal, with Million being one of the most famous parlors within the country. Many places outside of Japan make gambling entirely illegal so it isn't likely that fans who only dabble in anime would even think that such a genre exists if they weren't familiar with that aspect of Japanese culture. It's also more likely that fans within the country would also be drawn to the gambling aspect if they themselves love to gamble.
While the series was popular among a niche audience, it wasn't until big-name celebrities gave it more of a platform that it came to the mainstream. It's not uncommon for something to become popular because someone famous is talking about it -- people want to be like their idols, so if their idol likes this anime, then it has to be good, right? Social media also plays a role in whether a series will garner a foreign audience, with fan art and cosplay being able to be spread easily.
With tons of new titles coming out each year, it's fairly common for some to go unnoticed. It's hard to keep up with every anime that comes out in a season anymore, even for the most avid fan. Less avid fans are even less likely to keep up with the barrage of series that are released. Unless a series becomes popular right out of the gate, it's not likely to get noticed by the mainstream audience, which is why Kakegurui was lucky to have gained the boost it has.