I definitely don’t think there’s any single factor. Several of these are intertwined (see Game of Thrones and CRPGs for instance) which in combination have created a critical mass. In no particular order…
Its worth repeating point one, that it is a damn good game though. Were that not the case I don’t think we would be where we are now. Not perfect but a solid 4.5 stars. Better than anything we’ve had before.
- Its good. Its hit the sweet spot between simplicity and complexity.
- The production values are excellent, books feels like something you can give as a gift
- Similarly the quality of art is frankly mind blowing now. Characters, maps, vignettes, covers etc.
- It’s cheap at a time when everything else is more expensive. Take your family to the cinema once or play D&D every weekend for a year.
- Focus on the Adventure as the driving product. A build-your-own-package not a tool box and raw materials. At a rate which allows supply to keep up with demand.
- The rise of VTT has removed a huge historical barrier to play.
- Online resources - google images, patreon, pdf reprints, wiki’s etc have put an an astronomical amount of game resources at peoples fingertips for very low cost.
- it’s a broad church catering to a wide array of gaming styles.
- Lack of product bloat making it evergreen and reducing edition wars and fragmentation.
- Ubiquitousness. Imitation is the sincerest form of flattery (Looking at you Level Up)
- A generation raised on excellent CRPGs Skyrim, Final Fantasy, Witcher 3, etc. primed to play D&D.
- A generation folks who played the game in the 80’s and 90’s are playing it with their kids/grandkids.
- Inclusivity. The removal of chainmail bikinis, all-white artwork and their Ilk. Opening up the game to 50%+ of the population.
- The establishment of fantasy as a mainstream TV and film genre for everyone. Thank you Lord of the Rings and Game of Thrones.
- Geek-Chic reducing the disincentive to be seen playing or to suggest it.
I was just about to post much the same thing, so thanks for saving me some typing!

- People crave an excuse to have social interaction that goes beyond sharing videos or posts online. D&D gives people a structure to get together and spend time with people interacting with each other. At the same time, it's relatively low pressure in an age with polarized opinions on politics, religion and other cultural aspects.
- Online streaming has shown what the game really is like. That a large component can be people just playing, joking and being a bit silly.
- The plethora of resources in blogs and forums like this for when you have questions on how to do something or how to run a game.
- The generic nature and broad types of games that can be played to suit people's tastes.
- The flexibility to make small tweaks to the game without worrying about breaking the system.
But I also don't think we can underestimate how much the design of the game has helped. Yes, there are many other factors that means D&D is best selling, but that wouldn't have happened without a game that simply works for a lot of people. Much of the rules and text of OD&D was confusing, the math of AD&D was obtuse. With 3.x there was a tremendous difference between character builds that really raised the perceived cost of entry. The difference between an optimized PC and on played by someone more casual was huge and ubiquitous. With 4E, it was just a different approach to the mechanical structure of the game which simply didn't resonate with a lot of people along with the complexity of tracking conditions and interrupts along with hours-long combat.
Now we have an edition of D&D that takes the core concepts, cleans them up and packages them together in a comprehensive well designed product. It's not perfect of course, it will never appeal to everyone under the sun because it can't, no one is saying it can't be improved. But it is a game that simply works for a wide swath of people, including those that I never in a million years would have expected to enjoy playing the game.
Despite claims to the contrary, D&D does not have a magic hold on the imagination of people. There is nothing stopping another game from grabbing the crown. You can't just slap a D&D label on a game and expect it to succeed. The game has evolved over decades and, hopefully will continue to evolve. But I think 5E is the best version of D&D yet for broad appeal.