Even though Pokemon is a game created for children, not everything is that simple in its world, and neither it is Pokemon World Online. You see; PWO is set 8 years after the events Pokemon Gold and Silver (it's because PWO came into existence 8 years after Gold and Silver were released, in case you were wondering), therefore placing it in the Original Timeline... Pokemon has different timelines and dimensions, crazy, right? But it's all true and canon. This timeline is characterized by the facts that the Pokemon have no abilities, there's no physical/special split and no mega evolution among other things. This was the case for several years, but about 10 years ago something started to happen; the boundaries between dimensions weren't as clear-cut as they used to be, and soon an other dimension was making its way for this one: first Ghost type became special, a physical/special split followed soon thereafter, Pokemon were able to learn new moves through different ways than before: the two dimensions were headed on a collision course with each other... and they did collide: for an extended amount of time PWO was in a state of perpetual limbo where both dimensions existed while none of them did as well. After what seems to have been an eternity, the two dimensions drifted apart, but not without having shifted places; cause Pokemon were starting to look differently, gotten more new moves, and also became able to use abilities; cementing us in the Advanced(/Main) Timeline, another timeline that lacks the ability for certain Pokemon to mega evolve. Not only Pokemon felt the impact of what happened, but the world itself seemed to have changed as well: new religions formed and new landmasses popped into existence to name a few changes. Things seem to have quieted down lately, and the dimensional borders are appearing to regain their strength and stability, but who knows what the future has in store for us. So if anyone ever wonders why there is still no such thing as mega evolution in PWO, you can belittle them for their lack of knowledge about quantum physics and ask them why they would want to incite an all-out war.
P.S.: Even though this version I'm telling you today is factual, the official lore and reasoning might not be. Just keep that in mind.