I get where you're coming from. However, I'd argue back that a specific NPC's and especially an "end boss" and iconic NPCs performance in the story isn't just about his on-screen performance, since the players presumably learned a lot about him through the adventure prior to that point, thus a more interesting backstory indeed adds to the quality of the adventure. Besides, I as a GM could do more work with a more interesting and relatable villain, it adds depth to his personality, to his goals and generally makes me more interested in him. I like when an adventure has interesting background besides the interesting here and now encounters. Makes the whole more lively to me. that, and i like to read stories, I like to read the adventure background in Paizo's APs, because even when the players won't explore the whole of it, it adds to my game.
Because it was advertised as a toe-dip in Ravenloft. Because Ravenloft isn't just the adventure, it's the setting. They would just need to write certain things that was written any way, but not in the way of discarding the setting, but working with it. That and adding a paragraph in the introduction, or in the end as a "how to continue the campaign" section. In a perfect way a short appendix, like in the end of SCAG about how to integrate the class options onto other settings. 1 or two page at best.
Would it have been really that much? That much space, that much effort? Wouldn't have been better to acknowledge the favorite setting of countless fans and one of D&D's most interesting worlds? To really bring it into the new edition, to open gates instead of closing them? Why it worked in the past but wouldn't have work in the present? Would it detract from the adventure?
No, it wasn't made that way because it would have been that hard, or eating up that much page count from the book. It was made that way, because they didn't want to acknowledge the setting. Strahd's story was altered, because the Hickmans wanted to make a point. The adventure rejected the setting, because the Hickamns and/or the D&D staff, or just Perkins doesn't like the Ravenloft setting (I recall something from an interview, or discussion, i definitely remember reading somewhere that Hickman always disliked the setting). Maybe it weren't them, after all RL remained as the 4e paradigm, so might be that someone disliked, or didn't care about the old setting, or just like the 4e version better. Maybe because he/she worked on that, I don't know. I just think it was a bad decision. Maybe it doesn't matter to a lot of new fans, who never knew the setting, or fans who never cared about RL, just wanted to punch Strahd in the face. However, it matters to me and to a lot of people.
Its simular to how they have dealt with FR, the setting may as well not exist outside of the Sword Coast.
So using this thinking, Ravenloft is reduced to a wannabe Barovia, just as if they do an Eberron adventure it will likely be restricted a small region of the setting with zero support for the rest, if they do darksun the setting will likely be reduced to just one city, and so on.
This is a trend I dislike strongly. Trunckated settings, yuck.
As for Strahd I was fine with him being a really evil vampire, until I read the sexist logic behind it, which turned me right off. Villifying the male gender, and dumping the negative actions of the other onto it was repulsive to me, so I desided CoS was no go for me, period.