Let me start by pointing out that I thoroughly dislike the time jumps and the global disasters of 4e and subsequently 5e. I like 1e/2e/3e FR, but still run 5e FR in our current campaign
1. Why
Well... I could go with the basic answer of standard high fantasy world that's currently supported well and Nostalgia. And that's partially true, but I suspect the answer lies deeper. We started way back in AD&D2e, I remember back in 'high school' that I was making my own world maps and our own world, in the 90s I was buying what I could (afford) of 2e, every setting, every supplement. We did do some trips to Spelljammer, Ravenloft, Dark Sun, etc. But we used our 'own' world (which was a D4 sided world, one of the sides was Dark Sun). We kinda stopped playing AD&D2e at the end of the 90s, we made trips into Vampire, Shadowrun, board games, card games, etc.
By the time Eric Noah started collecting the 3e rumours, I started planning a return to D&D3e. By that time I had a fulltime job, just like the rest of the group, so time was a LOT more limited. Over the years I had collected most of the FR products physically, but had all of the pdfs (the official ones), I had also the FR digital Atlas, Dragon Archive, and made a pdf index file for all the FR pdfs/products (which was a LOT). So this was the most fleshed out setting I had complete (digital) access to. There was also a TON of fanmade material, there was quite a bit for Greyhawk as well, but as we all had read quite a bit of the FR novels, played the FR computer games (especially Eye of the Beholder in our group), I settled on FR for proto-3e. And used the fanmade Northern Journey project as our campaign, as I was already spending enough time stringing together 3e and 3e monster conversions.
D&D 3e brought us a pletora of good FR supplements, we continued playing in FR during our 3e period, most of us bought evert FR product and we are all very familiar with it. We skipped 4e completely, and we started 5e for a short while, then quit a couple of years and got back to it a few years ago. One of our group was DMing and we started in close to Thay, our next DM moved us to Neverwinter with the same party. When it was my turn to DM, we started a new 1st level party, I had a few campaign options available, one of them was the alpha version of Foundry VTT Ember (setting/campaign/sandbox), the other was in FR Undermountain, we opted for the later. My idea for that didn't start out as FR Undermountain, but as a mega dungeon I fully made, but I still have a fulltime job that is even more demanding then 25+ years ago, buying a house, having to run it in a VTT, I started looking at existing mega dungeons, like Rappan Athuk, etc. But eventually settled on Undermountain by way of Waterdeep: Dungeon of the Mad Mage.
So why exactly FR? Because we're all very familiar with the setting, especially Waterdeep as it was our home base in 3e for years. Why introduce a new world people have to learn all over again when they have demanding day jobs and families. Some of the group have a strong dislike for digital books, so they would have to buy physical books, etc. There's also a huge pile of existing official and fanmade material to delve into. So partly it's a bit of nostalgia, but I suspect it's also very practical, atleast for me as a DM.
2. How do you approach the setting (everything is in, stick to one edition, gray box only etc.)?
As a group we decided eventually to stick to the FR 5e timeline, not my favorite choice. But due to the lack of deep 4e/5e material we still often reach into 3e and 1e/2e material, either for history or more practical for maps and more detailed situations that the 4e/5e material didn't touch all that well or deep.
From my particular use, the Undermountain of 5e Dungeon of the Mad Mage is very limited compared to the 2e boxed sets... So I can easily use the maps from 2e for expanding the map, possibly using things from those earlier editions, sometimes with modifications for our particular adventure.
3. How long has it been your campaign setting of choice?
25+ years
4. What are your favorite game supplements?
I think I used the 3e FR core book the most, is that also my favorite? I don't know, I really like a lot of the 2e boxed sets as well. It really depends on what part we're playing in FR.
5. Do you like any of the novels? Which ones? And do you use the novels for game material?
I read a bunch of them back in the day (I would say pre 3e) when my choices were more limited in English fanatsy/sci-fi material. Which ones specifically, I honestly don't remember, but it wouldn't surprise me at all if all of the pre 3e (or 1e/2e) novels...
Did I use it as game material. Not really directly, more to get a feel for what FR was like, big events, possibly NPCs they could meet if appropriate. But often more as inspiration, I would often first delve into the actual FR game books, before actually using them.