OPM means Other People's Megadungeons.
There are not a huge amount of megadungeons out there, ready to play.
I think that beyond the logistical challenges of publishing a megadungeon, there is a deeper issue:
It is not easy to run someone else's megadungeon.
One reason Felltower is so easy for me is that I wrote it.
Bits on the map? I put them there. I can't run my dungeon without the maps, but I can picture the maps in my head. That's where they came from in the first place.
Factions? I made them up.
Room descriptions? I know what's supposed to be there.
Everything I have down on paper is a reminder. It's not new information. Even when there is a blank, I know what I was thinking when I wrote it and what would thematically fit. I don't need to roll on the "What does the faction do?" table. I know what they'd do. I might flip a coin to find out which option they choose out of two, but I know which two and why.
Contrast that with a published megadungeon. I've seen it with Erik trying to figure out what the heck something is intended to be during out own B-Team delves. You can see it with Jeffro having to puzzle over room descriptions in Dwimmermount. I've had it with reading Barrowmaze and Castle Zagyg to see what is in it. You need to become intimately familiar with someone else's work. Not only that, but they need to have written in a way that's accessible on a read-through, clearly communicates intent, and which is easy to use in actual play.
You get that with any published adventure or setting. But the scope of a megadungeon can make this so much harder. I've read Stonehell through a couple times, but I don't know it so well that I could run it without notes. I could run my megadungeon with just the maps, if I had to.
That's tough. It's a problem I don't have with my own megadungeon. And it might be yet another reason there aren't a plethora of them out there, why playthroughs of them don't seem to be very common, and why you can see a big of struggle from GMs running them even if they otherwise run games smoothly and easily.
It's the OPM problem. It's not yours, and it take a lot of effort to really own something that big.