Or more ignorance.
Not true at all. About the modules any way. There are a TON of modules. More than there were in AD&D for the same time period. Not only that, but it says right there under the Adventure League page at WoTC that you can play adventures you find at the DM's Guild. Also, I've even run my own megadungeon at an AL event.
Not exactly. The modules sanctioned for AL play are available at the DM's Guild - that's where you are supposed to purchase them. But they have to be sanctioned - not all the modules on DM's Guild are available. You can't just run anything that is thrown up there - that is why DM's Guild has a special section for Adventure League.
The reality is, the vast majority of the modules run are the "official" AL modules associated with the 6 seasons that been released so far - each season associated with a hardback Adventure book from WOTC. There are also some modules from Baldman Games that are sanctioned, and not associated with the Seasonal storylines.
The reason is quite simple - DM Awards are vastly higher for running the official mods and adventure books than for running anything else. DM's can get DM awards for running non-AL mods or even homebrew games, but that's just for the DM's. You can't play official AL characters them - they can only be played in official AL mods or the hardcover Adventure books (or Epics - which are mainly run at conventions and large game days).
I wouldn't know about your "megadungeon". Either it wasn't official and the players couldn't keep any XP, gold, or magic items from it on their characters, or it was a one-shot with pregenerated characters, or the event organizer's had special permission to run it. Since I don't know the details, that's the best I can guess.
So no, there aren't a limited number of modules, and I'd back up what Zardnaar said earlier. If you have people choosing to play an adventure because of what they know they will get, that leaves a huge bad taste in my mouth.
Your opinion has been noted. Might I suggest a mint? Because it's happening whether you like it or not, I'm just the messenger. But feel free to join Zardnaar in calling me names if you like. I'm used to such hypocrisy around here.