I think the issue with many of the WotC mega-adventures is that they are written by teams. They are designed to appeal to wide audiences, truly pleasing no one. They are sketched out with multi-format marketing campaigns (video games, novels, audio books, miniature lines, etc.) to tick all the boxes.
A shorter adventure could be a passion project of a focused writer (or a very small, coherent team). Instead we are getting the equivalent of adventures that resemble the worse traits of Hollywood blockbusters - overlong, passionless, soulless sequels and reboots.
People on here are claiming that 5e is a golden age of quality adventures. I don't think so. Compared to some of the classics from B/X, 1e, and even 3e (not to mention some of the interesting 3PP stuff), I'd say the 5e stuff is worse.
Why are 5e not as good in my opinion: they are too long. They dilute content, they don't tie together plots, they let good ideas stretch on to the point of nausea.