Because there are two seperate teams working on WotC modules, a story team and a design team.
The design team makes a actual adventure that is more open ended and supposed to be a broader scenario, then the story team adds a odd "story" too it, that usually adds a pretty linear plot too a more open ended thing, with tons of odd rails and stuff to make it a "story".
I've noticed this and its bugged me every since, and notably curse of strahd and a few others which dont have the story team trying to rail it, are some of the best out there.
Utterly untrue.
Story comes first. Once the design for the adventure (the story and structure) is worked out, then the designers go about putting it all together. Things get adjusted as it goes along, sometimes not well.
I believe that the most problematic Wizards adventures have been those with a large team of designers (mostly freelance), because writing just one section of an adventure which needs links to other sections, when you don't know what the other sections are, is terribly difficult. It requires the Lead Designer to stitch everything together - and this hasn't always worked well.
And sometimes it hasn't worked so well that there's been a lot of late changes. I believe
Descent into Avernus was originally a lot more freeform/sandbox in nature, but things did not go well, and a more linear plot was imposed - not all that successfully.
The less linear an adventure is, the more tricky to write. I think
Rime of the Frostmaiden was an attempt to do as sandboxy as they could go while still maintaining a story. I think its problems derive from that and the "not a small team writing it" issue. Though I haven't run it yet, so I don't have first-hand knowledge of its play.
I do find
Rime interesting because it, with its multiple threats, hearkens back to
Legacy of the Crystal Shard, an incredible late-4E/5E-playtest adventure. But where
Legacy is something I recommend designers study, due to its innovative design and structure,
Rime didn't quite recapture the magic of the earlier adventure.
Cheers,
Merric