The longevity of a game depends on too many factors to accurately pinpoint a cause or if one game has more than other. Let's try to do a breakdown:
QoL
NH "grind" will never be comparable to all the QoL features that brought to the series that have made it faster and more convenient than other games. The simple fact you don't have to micromanage to hell and back to use the measly 15 spaces means you can do more things and those usually offset say, breakable tools (of course, people never mention this because focusing on the negatives is what cool kids do). You can buy 9 rods, fish 30 creatures before it breaks, instantly warp to the store to sell (thanks to the pipe) or instantly store everything (either pipe or storage shed). On previous games you could only fish like 15 creatures before having to start releasing or running to Retail to sell/house to store (well, only running if you're not worried about grass deterioration or trampling over your flowers, otherwise, walk). Being able to store thousands more things also deletes the micromanaging aspect, so there's also less time spent there. Do you need to buy clothes? You can instantly buy up to 8 things while you try everything in seconds, instead of having to try a single item one by one, listening to Mable/Label say the exact same thing and buying one by one.
All these improvements mean there's less time "wasted" on mundane things that happen daily. That's not to say there's other QoL issues on NH, but the pocket space and storage space are just enough to speed-up the game considerably. All games have things that pad the time played and it's frankly, a bit ridiculous to imply NH is objectively "wasting" more time than the others lol
Different games, different features
You like going online to play minigames with other people and find them amazing and not repetitive? Congratulations, you'll spend tons of hours on New Leaf. You like designing a whole island with the biggest amount of options ever and find that amazing and not repetitive? Congratulations, you'll spend tons of hours on NH. Both activities are features with infinite potential (curiously, both don't involve villagers/residents) but it totally depends on your tastes. Previous games didn't have a similar infinite one to my knowledge but I guess the constellations from WW/CF and the ability to directly ask for favors of the original kind of fit the bill.
All games feature some kind of RNG hell (DIY for NH, PWP for NL, etc.). NL has 3 microgames to spend time with (2 of them only available 4 years later), NH has a whole other game to spend time with (money issues may arise tho), all games feature events, a collectathon aspect, villagers/resident to interact, etc. The point is, since every game has different features and content available, the time spent on the games heavily depends on what the player likes to do/prefers.
You can see this in full-effect when people talk about grind. Grinding for the game they like is lovable, charming and rewarding. Grinding for the game they dislike is a chore and an artificial way to lenghten the game. Grinding bugs and fish for bells on Tortimer Island is fun and rewarding, grinding for materials on your island is a hassle. Grinding for PWP is engaging, grinding seasonal DIY is poor game design. A grind that you like/don't mind is time well-spent and a contributor to the longevity and a grind that you don't like is a waste of time and means I drop the game faster thus no longevity.
Time-based barriers
If you actually take the time to count, you'll notice NH has a similar amount of unlockable things than NL, the game that had the biggest amount of upgrades, etc. But a key difference is that NH seemingly offers the player many more things faster than its predecessors. It doesn't matter that the Cranny on NH offers more items overall, you can get the upgraded Cranny on NH on 35+ days, whereas you need 70+ days for the one on NL. The museum shop needs 14 days of speaking to Blathers while you can open the 7 different caravans on Harvey's on 7 days. These kind of things create a better longevity for NL, since you need to wait more in order to get more, while NH on the other hand, just gives the player tons of things almost instantly. (of course tt makes everything moot)
External factors
The longevity of NH will be forever tied to the pandemic. No one can seriously try to discuss longevity without pointing at the biggest elephant on the room I have ever seen, as the pandemic completely took a toll on the way people played. Everyone can look up social media and notice how many hours were spent on the game merely weeks after its release compared to other games. Of course, that means burn-out is/was at an all-time high; which also affected how people played the game.
Social media and content creation also plays a key role, tons of people were rushing to burn through all the content either to upload the prettiest islands, to get the views on YouTube or Twitch, to create the guides with all the datamined information the instant an update came out, etc. Nobody wanted to lag behind, and with 15m sales on the first weeks, that means there was a rush to go with it. Nobody wanted to wait until December to catch the stringfish, everyone wanted to catch it on the last 10 days of March. Nobody wanted to wait a year to get the full-set of Cherry Blossom DIY, everyone wanted to grind balloons for hours for 10 days. And the pandemic meant people had the time to do all that.
And even when the pandemic and lockdowns were not in full-force, people still did this. Just look at what happened with the 2.0 update and HHP update. People rushed through an insane amount of content in days. Some played too much in too little time and it shows.
So yeah, there's too many things to consider. Not even counting that we also don't have official data for engagement, so lots of the things surrounding longevity are tied to anecdotal evidence ("NH fell faster than other games"/ "there's less people playing than x other game"). At some point, it was clear that the amount of content is not the biggest factor, but what people enjoy and what they don't, and how they react to that. It doesn't matter if NH has more content than NL/WW/OG if you simply enjoy NL's/WW's/OG's content better. That doesn't mean one game is inherently better than the other, all are life-sim games that you can sink lots of hours on them.
But the shadow of the pandemic will always loom over NH and for millions of people, that was key to burning-out faster.
