The dirndl hasn't actually been confined to Oktoberfest for a couple of years now - it's established itself in fashion and festival scenes. For one thing, the fashion industry has been reinterpreting and hyping up traditional clothing pieces. Designers have started creating modern, more flattering versions that don't look like "grandma in Berchtesgaden" and work more as statement pieces. At the same time, there's a bigger trend toward regionality and craftsmanship that a lot of people find appealing - it's not just cheap fast fashion off the rack, it's something with history.
Then there's the social media component. Instagram and TikTok have massively popularized the dirndl because it's just photogenic and lets you stand out from the average festival look. People post their outfits, others get inspired by them, and suddenly it becomes normal to wear a dirndl outside of Bavaria too. Festivals have picked up on it and some have even actively pushed it themselves. What also matters: it's practical. A dirndl looks elegant, you can dance in it, it's comfortable, and you don't have to spend hours thinking about what to wear.
Whether this is just a trend or here to stay remains to be seen. But right now it looks more like it's established itself rather than being just a passing fad like some fleeting hype. It works too well to disappear again.