1) Why was geek-stuff going mainstream all of a sudden (techo-philia due to phones?)
2) Why has gritty (seemingly) fallen out of favor now after a decades-long trend? (e.g. Miller's 1985 Dark Knight --> DC hero-films)
3) Why are people turning away from computer games towards board-games? (tech burn-out? lack of social alternatives in a isolated digital age?)
4) Why are so many girls suddenly trying D&D for the first time? (geek-sheik?)
5) What are the elements of 5e that make it particularly newbie friendly? (first I've heard of that)
As best I can tell:
1) TV shows like The Guild or The Big Bang Theory (which has fallen out of favor now) used to be insanely popular. YouTube productions like TableTop and Critical Role brought more rookies into the fold. Geek-chiq was a genuine fad ~7 years ago. D&D being on Stranger Things, Community, and other mainstream outlets brought the hobby out of the shadows.
2) It became cliché. When a supposedly light-hearted hero movie like Superman becomes gritty for the sake of grittiness, then people stop caring for it. It worked with Batman and Deadpool, but it's not original anymore. I mean, Drizzt was once a pretty cool idea, but nowadays people would just accuse him of being some tryhard edgelord character.
3) There was a tabletop game renaissance a few years ago, right around the time Settlers of Catan and Munchkin got big. Board games are more social, and geek-chiq only worked if the geek activities were more "normalized" to society (ie. everyone has played board games before). It was something of an easy "baby step" for people dipping their toes into geek hobbies, because it was so approachable and a social thing with friends.
4) D&D is mainstream now, and half of that "stream" is women. There's nothing inherent to D&D that ought to be turning women away from it, so it makes sense that they too would be included in the new flux of players. Also, there' something of a "I'm so quirky, I even play D&D!" appeal that ladies like (see also: I'm a gamer gurl xoxox!)
5) The rules are condensed. Everything is "roll a d20, add a single modifier" for the most part. In other editions there were way more modifiers and conditional things (Ranger gets a +4, unless it's an ooze, in which case they get another +2 bonus, plus an additional modifier if the player succeeds at this skill check, plus the modifier for using this feat, etc.). That shit didn't jive with casual players and newbies. 5e makes it smooth. Situation is beneficial for your action? Roll TWO d20s and use the higher number. That's about as complex as it gets (barring some more situational class rules like wildshape and whatnot). People are quick to adopt what they can easily understand.