In my opinion, the reason why modules don't sell well is because of two reasons:
1. Wizards doesn't put enough history of these places or their associated places into the modules themselves. They expect the DMs to already know all about the history via other sources that aren't available anymore or from other iterations of DnD itself.
2. DMs of this generation are expecting everything handed to them and to not have to do any sort of work in order to actually make the module work and be fun. They get pissed when they have to actually do any form of research outside of the module itself to run the adventure.
Back in the day, modules were maybe a total of 28 pages, maybe more, and I honestly haven't heard too many complaints about them being lacking in information or being boring, but I hear far more complaints about the newer modules that are 100+ pages. This tells me that modules back in the day had more history in them than the ones now, and that DMs today are expecting to have everything done for them without having to actually think even with their 100+ page modules. This could easily be solved though in two ways:
1. Print books like Van Richten's with more of the history of these areas that a DM could pick up optionally to run a campaign. This would encourage people to pick up the modules if they knew they could also pick up a book that has the area's history and whatnot. I mean, this would only encourage Lazy DMs to keep being lazy, and honesty I don't think they will ever stop complaining regardless of what is published. With that said, at least Wizards could point to the supplement and say they have provided it all in one place instead of having to scour the internet for everything. Having modules already 100+ pages and then trying to add history into that would make the books 200+in some cases, which are heavy and clunky to play with really.
2. DMs need to accept the fact that being a DM takes work, work that you might not be ready for and you should really consider that when you take on that role. DMs have more tools now than they did back in the day, where it was literally only graph paper as a resource and one's imagination if you wanted a map that wasn't printed in the module. If you don't want to do a few extra minutes in either coming up with a roll table for encounters, or look up on the wiki pages about the place you are playing in, then DMing is not for you.
I'm not saying DMs need to pour all their free time and energy into looking for information and fleshing out modules, but that's the entire job of the DM is to do that, it's been that way since the inception of Dungeons and Dragons; it's up to the DM to do the work to make the module run and make it fun. WIth that said, Wizards needs to consider making the history of these places more accessable and less scattered throughout different interations of DnD as well as their modules that no one can really purchase anymore. They could print a book of the history for the peopel that want it, or they culd make one centralized website with it on there so that DMs can reference it.
As for 3rd party things...I don't tend to purchase those, those types of things make me a bit leery. I had a bad experince with them (honestly probably would blamde the DM on it as it was one of my first groups that wasn't my family lol) so my opinion on them is a bit biased. There ar epeopel on here far more versed in 3rd party things than I am and probably far less tainted by bad experiences than myself with them lol.
