Much like Lord of Verminion, Mahjong is too complicated for casual players to learn.
And much like Lord of Verminion, it will probably be dead in 2 weeks.

1. The game is too complex.
This could have easily been fixed by starting novice matches with limited rules, and adding complexity at each tier. With challenges & incentives (rewards), people might at least be willing to invest time into learning the game.

2. No tutorial.
Why wouldn't there at least be a tutorial that holds your hand and explains the game? The text descriptions in-game are beyond confusing and unclear. Which is why multiple people have created online documents to explain the game.

3. Innate unfairness
New players are unlikely to ever beat players who have a lifetime of experience. And with a high-stakes point-based ranking system, new players are just going to get wrecked over and over and over again

4. Time
Hour+ long multiplayer matches are insane. And the payoff isn't amazing.

5. Bad automated suggestions
The automatic suggestions kind of screw up the entire game. Because instead of playing and learning the game, people just begin to rely on suggestions. And then second guess themselves when they feel like making a move different from the suggestions.

Why does every major minigame in the gold saucer have to have convoluted rules, or pointless time investments. Triple Triad is full of complicated rules that aren't necessarily easily picked up. Chocobo racing has an entire breeding system behind it. Lord of Verminion requires a bunch of strategy. And Mahjong is Mahjong....

Why can't we just have a minigame that anyone can pick up in seconds. That is fun. And that people actually want to play on a regular basis for the sake of playing it? I would bet if they added multiplayer pong, more people would be playing it that any of the existing games.