Well this one is fairly easy. There are a bunch of games I am not playing right now that might get played again in the future, so I am going to take this as "... no longer play and are unlikely to play again." That does make it a lot easier.
I spent a lot of time playing AD&D in the 80's. We had a blast and it was the major game that formed the baseline of what "RPG" meant to me. I bought the books, read the magazines, painted the miniatures, wore the t-shirt, and fought through innumerable adventures for a good 10-11 years, from elementary school into the college years. When the next edition came out we moved on to it and never looked back. That's sort of the problem ...
I haven't sat down to run a 1st edition AD&D game in over 20 years. I haven't played one either. I loved this game when it was "the" game but I've never thought the mechanics were the peak of game design for all time. Everyone I played with thought 2E was an improvement and moved on. We all moved to 3E when it came out too.
When it comes to nostalgia I've considered running an old school game but when it comes up B/X D&D is the game of choice as it's simpler, cleaner, and faster. Goblinoid Games managed to add in most of the good parts of AD&D into it with their Advanced Edition Companion for Labyrinth Lord.
So there's not much need to go full-retro with AD&D just to be able to play a dwarf cleric. I suggested it to the Apprentices and they said "but we like Basic" and that was that.
For a while I even considered running Hackmaster, the older edition, which is a whole bunch of AD&D 1st & 2nd edition with some updates. No one took it seriously as an option.
While I may have some nostalgia for it, no one else is willing to spend some of their limited gaming time on an old, clunky game when there are other options, options they like better, available. So it seems unlikely, very unlikely, that I will ever run or play this again unless I stumble into a nest of old-schoolers - and decide it's truly worth some of my limited gaming time.
We had some really good times playing this game in the late 80's and 90's. You wouldn't think the end of the civilized world would be great for that but it was a lot of fun as a game. It was never the most popular RPG but it was my favorite game of that type.
This one is suffering from a similar yet different problem from AD&D. Tastes change and time moves on and my friends and the Apprentices are not terribly interested in the post-apocalyptic game as a genre anymore. With AD&D it's about the mechanics - the subject matter is still awesome, but the system has been surpassed. For Twilight 2000 the mechanics are fine, and being able to cut loose with an automatic grenade launcher from your humvee is still a cool thing, but the setting as a whole is just not something they care about.No magic, no mecha, no powers, just normal humans and real-world gear. I'd love to run some of my current players through the epic published campaign but I doubt that will ever happen.