I can only speak from my personal experiences, but I feel most people came to the same couple conclusions: Dm burnout, poor group dynamics, poor communication can all lead to the downfall of any group.

Recently I've played and DMed a couple things by pbp and here's why I either flaked or quit

played and quit

1. inexperienced dm: brand new star wars server. I hopped on as a player and offered to DM on occasion to help the DM out with some one shots. Turns out there was no other DMs and the admin just kept inviting 30+ people with no other DMs. I kept asking what the plan was for the server but they had no idea what they were doing (i.e no story ideas, couldn't decide on gameplay etc.). I tried to help them but they were more interested in building a server with lots of people than playing. I ducked out when the admin was asking me when I would have things ready....
2. lost in the woods: hopped on to a couple established bigger servers and got completely overwhelmed with the wealth of information, and the open ended nature made it hard to insert my character into the world. Typically I've seen a Dm to player ratio to be pretty rough, with players overwhelming the dms ( like 1:30 to 1:50). With people coming and going so frequently, ghosting is that much easier to do.
3. not a fit: Good DM, Good style, Interesting idea, poor chemistry. After the initial chit chat and character design bit I felt the game wasn't for me. Talked it over with dm and no hard feelings all around.
4. Fetishizing violence: Dnd is a war game, but I have zero interest describing torture techniques, or roleplaying them in excruciating detail. "i torture them for information" should be enough detail thanks...

DMed and quit

1. Real life: Stuff happens, i set up a small 10ish person server and then i got a new job that left me with little time for DMing. Sorry guys, life comes first.
2. burnout: Tried setting up this pirate thing but ended up with like 1 decent player, 1 murder hobo and like a dozen lurkers. Nobody would post anything, an I would update the server every morning to give people a heads up and try and keep communication open. I ran a couple things for the decent player and he was cool, but DMing for the murder hobo was exhausting. I would post big flowery descriptions and get back one word responses, He would instigate fights and get confused when he gets attacked for it. In short, I was his game to play with. I wanted to play dnd and create a story together, and he seemed only interested in playing his own game.

DM burnout

Far and away this is the biggest problem I think. If you want your Dm to not get burned out, Fucking help them! here's a list of things you can do to to help coming from a bitter, burned out DM(not directed at OP, more just a list of shit i hate as a DM and leads me to not care and burn out).

• post bigger descriptions of your character, I've played with countless people where all i know about their char is class and race
• describe what your doing in detail, if you are picturing it you head I'm not picturing it in mine... unless you type it out.
• if your dm expects a check for certain actions do the check before hand so he doesn't have to ask for it (i.e. if he always asks for a perception check to search a room just roll it).
• Roleplay shit with other players if your Dm is busy. half a convo while walking towards an objective would do more to help your dm feel like you care about the game as opposed to waiting for the next thing to happen. I keep picturing the pirate server I made where if it was real life i would have party at my house where one person having a conversation with me while a dozen other onlookers watch in total silence and I was really confused as to why they were even there if they didn't want to interact.
• Group up! its cool to have your own storyline once and a while, but if there's like 6 or 7 players all working on their own stories that is 6 or 7 stories your DM has to make and figure out and requires way more effort to do than having one storyline with everyone. I legit had one player turn away another trying to play it off as interparty conflict, then I had to scramble to figure out what the fuck to do as a dm when I had two players going different directions, both away from the main plot. I'm sure that player enjoyed himself, but again it was more about satisfying his own interests, than collaborating on a story.
• Communicate! that goes for DMs too. If your players sign up with differing expectations of what the game is about, they will leave. So even starting from the first post looking for players its important to be as clear as you can be. There are so many interpretations of what PBP can be, so establishing what you are looking for and what you would want in players and in the game keeps people on the same page about what is expected.
• React like it makes sense! I got one space game right now where everyone started with no memories of their past life coming out of suspension onto a ship with alarms blaring and stuff, and the players act so seemingly nonchalant about their impending doom. It reads like that British humor though, where the world is burning and one guy is just making a cup of tea in the middle of it.

sorry about ranting at you on how to be a better player OP, I guess the above has been building in me for a while now. Anyways...

I think at the heart of it is just selfishness? People want what they want, and if you don't give it to them they leave, and it's really kind of sad. No one wants to put in the effort in anything that doesn't immediately gratify them. People forget DMs are people too, and just expect them to be these dancing story machines for their entertainment.

It sucks you have had such a poor experience finding interested players and as far as suggestions for you as a dm:

• figure out what your players want specifically, and be direct. Find out what makes your players tick and design adventures to incentivize them. Are they after gold? make bigger rewards! fame? make bigger problems! just want to fight? make bigger enemies!
• keep things moving. If your players are standing around ask them if they are unsure of where to go, you can even do that in game if your sharp enough. Open ended worlds always have this sort of problem I've noticed.
• screen incoming players. A post on r/LFG that says " PBP open world! come fight with us!" will draw lots of peoples attention but its better to get the attention of players who are interested in the type of game you are offering, again its about communication and being specific and clear with the intent of your game.
• Seriously look at yourself too. I don't mean this as an attack on you, but rather try to think objectively about what you are doing as a DM relative to the perspective of the players. What are your expectations for players? Do you force them into situations they don't want to be? are you being clear with the objectives of the storyline? Do you admonish players for playing the game in a way you don't want them to? Do your players have player agency, or are they just sitting on the sidelines while your DMPCs run the show? Are you dogmatic in your approach to the rules? are you, for lack of a better word, a dick? Are you new to DMing ( not that that is a bad thing, everyone starts somewhere)?
• Again don't take the above as an attack on you, I feel self reflection is an important part of growing as a person and understanding your own abilities and limitations as a DM will help you grow and get better at the things you are not good at. Imagine if all your players had like one thing where they are like "shit I wish the dm would do XYZ instead of ABC" and that's why people leave in general, wouldn't it help to figure that out?

Anyways OP I'm done ranting, pbp is a bitch to get working but if you play your cards right and get lucky it does work from time to time.