There are many reasons. Without all of them playing a hand, a group crowned "4th gen queens" doesn't decrease its popularity while we are in era when 4th gen groups are doing better than most of 3rd gen.
1. JYP's creative direction- With every comeback, not only is their song getting more watered down and somewhat immature. Frankly, they just look/feel too old for their concept
2. The group- The members don't portray the songs being given to them well to the audience and do them justice with their vocals.
3. Visual direction/MV- I'll keep this short. Everything is subpar. It looks like a nugu group mv. Uninspired in everything.
4. JYP's strategy and itzy's assigned role that doesn't fit them- What JYP needed to do with itzy was even if korean charting wasn't very great, they only needed to stick to their plan satisfying global fans which they were doing really well. Even forget korea to focus on global. Yet this was practised by jyp on another group Twice and developed a strategy for itzy to focus on more korea which led to songs to appease koreans which don't even fit the group.
5. Nmixx- It's no secret this group's debut is hindering itzy from fulfilling their potential which many groups were able to unlike itzy.
6. Oversaturation of market- "Better ggs with better songs with more success to enjoy. Hell yeah".
7. Previous comebacks- It hurts a group's image if it's been a while since they have shown something good. And all year it's only huge misses. Maybe they should have considered aespa strategy to think more about what to show. The biggest rule in kpop that needs to be followed is don't comeback twice in a year that u underperformed if your product is nothing short of amazing. Sneakers and Boys Like You were exactly not that. One two knockout punch for itzy.