My Illogical Theory
Only one legendary has received a Mega Evolution, that is Mewtwo. I believe it's due to unpopularity since nobody really knows Mewtwo anymore. It never really made a major appearance in it's Kanto games. (The Legendary Bird Trio were part of the storyline and had to be battled in order to continue, if I remember.) If I am mistaken, another reason is that Mewtwo is found in Cerulean Cave - a place where not many people would notice, let alone go to. So, I believe that's the reason Mewtwo got a Mega Evolution.
Like KoD said, legendaries are literally gods (well, most of them anyway) and Dialga, Palkia and co. are that most - they are gods. So, they can't become 'more godly'. That would just be unfair.
Also, to add to this, Giratina already has an Origin Forme which Giratina gets access to when it gets the Griseous Orb. And Arceus has like 17 different forms (the plates and it's type and stuff) and Dialga and Palkia do have their own little item, but it doesn't create a form or anything.
So... yeah. By stating these things, these Pokemon will probably not get a Megavolution. Sorry.
Hope I helped. :)