I don't see how it's unfair. I think AsciiDoc is much more complex, by just about any metric you want to use. I'm not saying it's necessarily worse by many metrics, just that by the particular metric of getting average people to use it, and not just a random subset of it, Markdown's simplicity is beneficial.
> Nobody forces you do use all of them.
No, but for random internet user there's a real trade-off between what you're trying to accomplish and what it takes to accomplish it, when you're just trying to write a simple comment. Reddit has a link that says "formatting help" below the comment box, and when you click it, it shows every option you have for special formatting with examples in a table with nine rows and two columns, including the header. They could have chosen AsciiDoc, but then they would have to make a decision about what features to elevate to the quick help and which not to, and very possibly which to disallow for their use case.
Markdown is simple for users to use, simple for user to understand, simple for developers to implement, and simple for developers to decide about. That simplicity is both why it was adopted by developers and why users bothered to learn and use it. As I alluded to before, AsciiDoc may have been a better choice in the end, but I don't think it's quite as simple as AsciiDoc does more stuff, so it's better. It's all about the trade-offs, and there's been plenty of discussion on that[1] before, from both sides.
1: Just google "worse is better".