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It ultimately can be traced back Blaziken & Mega Blaziken being banned. Blaziken and his Mega Evolution were deemed too powerful by the head honchos of Smogon (even though it's not that difficult to counter them in the existing metagame), so they were forbidden from being used. This led to the already very powerful Mega Kangaskhan being ridiculously overused, as it was the next-best Mega and its most formidable counters, Blaziken & Mega Blaziken, were out of the picture. At the same time, the also already very powerful Mega Gengar became equally overused as Megakhan, as IT was the next-best counter/check to the mighty kangaroo and its child. Naturally, with these two being so powerful and so frequently-used, Smogon opted to ban them, as well.
I think you can guess what happened next. Yep, Mega Lucario, another already powerful Mega Evolution, started being resorted to more than ever, as it was the next-best in line after the previous mighty three had fallen. It's debatable whether or not he was as frequently used as they were, but regardless, he became too powerful in the eyes of Smogon. What they fail to realize, however, is that they MADE him that way. The rules and regulations they had set in place from the beginning is what led to the previous bans, and it's trickled down to Mega Lucario as a result.
You may be noticing a pattern here. That's because there is one. Every time a ban has been set in place, it's led to another ban, and it's not going to stop any time soon. This is going to keep happening like a butterfly effect until all the good Megas are no longer usable. Before long, we'll be forced to resort to Mega Ampharos and Mega Banette if we're to use any kind of Mega Evos -- not to knock on those two, they're good... just the weakest of the bunch.