In the case of a four-player high-difficulty dungeon, there is much less room for players to work with when it comes to party deaths or mistakes. If even a single party member is incapacitated or makes a mistake, it will invariably lead to a wipe, with not much leeway to save the run or come up with a creative solution. Another major reason is because our community continues to grow—naturally this is bringing in many casual players, and I believe that when looking at the bigger pictures, large-scale content will be easier for all players to get into
Basically it admits that they prefer large scale challenging content, because a single player playing poorly will not necessarily wipe the run. It's that mindset where even if some players want challenging content, they aren't allowed it because they're supposed to be carrying the casual playerbase and that comes first. It's a strange mindset since the Japanese community actively avoids carrying people who don't contribute to the point of unofficial public blacklists. It's also very unrewarding for the skilled player to be constantly putting in work carrying the casual playerbase, yet rarely receive recognition or their own content and are often shunned as a "tryhard" or "elitist" by the very players they carry. Why bother?

In an ideal world, we'd have both a healthy mix of casual and challenging content, including dungeons, and both types of player could enjoy their own content. It's a shame, tbh Mythic+ wasn't even that hardcore outside of pushing very high keys and they don't even have to adopt the timer system.