I believe that several features of Fifth Edition's design are responsible for its widespread appeal. I do not believe that speaks to design quality. It is a strong design that consistently delivers the experience it was built to deliver. I absolutely believe experience is the right word here. The designers were much more concerned with the cultural experience of Dungeons and Dragons then building the best game possible. Do not get me wrong here. I happen to believe they still built a fairly good game.
One of the design features that has led to the game's mass appeal is that focus on the cultural experience of Dungeons and Dragons. This could be seen pretty readily in the play test surveys. There were not many questions that were concerned with quality of game play, decisions, or difficulty. The overwhelming theme was if it felt like Dungeons and Dragons. Overwhelmingly they want things to feel familiar and comfortable, not just for existing players, but also for returning players. They want new players to feel like they are taking part in something ancient with a storied tradition.
Juxtaposed against this was Fourth Edition's approach. It was more like Marvel's Ultimate universe. It had an attitude and an edge. It was not your father's Dungeons and Dragons. It was your Dungeons and Dragons. Everything was different. The classes were different. The races were different. It was visceral and violent. The world was on fire. From a marketing standpoint this was a misstep, but it allowed for what was at that time some pretty compelling and innovative game play.
Another feature that has really helped the appeal of Fifth Edition is that it does not really reward deep engagement.
The difference between more skilled and less skilled play is fairly marginal when compared to other versions of the game. As a casual player you can sit down with a Champion Fighter and perform nearly as well as the more advanced melee classes like the Monk or Paladin. Neovancian casting is more fluid and forgiving to players. The Concentration mechanic and generally low rate of interactions between spells and other abilities serve to make playing a caster for the first time an easier experience.
The Beliefs, Instincts, Flaws, and Traits generally reward light characterization instead of deep protagonism. Like Fourth Edition before it Fifth Edition continues the trend of reemphasizing alignment and removing character constraints. This includes things like Dexterity to damage, rage having no meaningful downside, no penalties to attributes, and special ways to guarantee sneak attack damage.
This is helpful for players who just want to go on adventures and is less intimidating for new players. To that end the Organized Play experience for Fifth Edition is extremely streamlined. It is pretty much pick up and play. Scenarios are resolved in a couple hours and are generally tuned to err towards success with maybe a hiccup or two along the way.
The thing I find most interesting about Pathfinder 2 is that it has doubled down on deep engagement and difficulty. It kind of feels like it is trying to be the Dark Souls of the table top scene by rewarding deep engagement in both lore and game play. It has doubled down on things like encumbrance, alignment, spell preparation, monster immunities, and deep interactions between mechanics.
In many cases it has even gone further. Champions come in three varieties : Paladin (LG) , Redeemer (NG), and Liberator (CG). Beyond their alignment requirement each has a specific code with a matching set of Anathema, actions which can result in the loss of certain abilities until they have the Atone ritual performed on them by another member of their clergy. Champions and Clerics both have Anathema that depend on the god they serve. Barbarians also have an Anathema based on their Instinct, the source of their rage.
Pathfinder's new Bulk system brings encumbrance back as an expected part of play. Some classes even have trouble starting play unencumbered if they neglect strength in character generation. Exploration is also a big part of play with skilled use of Exploration activities having a strong impact on success in combat encounters. Things like searching specific areas both in and out of combat is an expected part of play.
While martial characters are only about as complex as a Battle Master the 3 action economy has put skilled play on the map for martial characters. Skilled use of actions, new martial focused consumables, special actions, combat maneuvers, and the like are necessary to succeed. Casters have had their spell lists revamped to build risk back into playing a spell caster. Now there are always a range of possible outcomes for any given spell and even effects like Nondetection, True Seeing, Remove Disease and the like require countering rolls. Timing and risk management are now a big part of playing a caster.
Monsters now usually have a wide range of immunities, resistances, weaknesses, and unique mechanics to contend with. Some like the hydra even have special ways you need to kill them. There is an increased emphasis on learning the monsters and becoming skilled at killing individual monsters again. They also inflict some pretty nasty long term consequences again.
Because of these features I am pretty sure that while it will find an audience it will never have the mainstream appeal that Fifth Edition does, but that does not mean that Fifth Edition is the better game. The more appealing game sure, but mass appeal does not have a lot to do with quality. Moldvay B/X is a far better designed game than Advanced Dungeons and Dragons, but AD&D had more mass appeal.
Again not saying Fifth Edition is a bad game. I think it's a pretty good game. So is Fourth Edition. So is Pathfinder 2. Fifth Edition would not be this popular with a discerning young audience if it was not good, but appeal is not just based on design quality. There are a lot of other factors that go into it.