I'm answering this. Yay!! :D
My Illogical Theory
As both answerers have already mentioned, the older Pokemon have not been seen for a while in any genre of Pokemon. For example, Mawile has only been seen in one episode of the anime (AG105) and it has barely been mentioned since.
It has also mostly been upgraded for most unevolved Pokemon, for example Pinsir and the aforementioned Mawile. They may have wanted these Pokemon to have a Mega Evo so they can say that they gave these Pokemon another evolution. x)
It's also noted that most mega evolutions are given to Pokemon with really stupid stats. A great example of this is (you guessed it) Mawile. It's highest base stats are 85, in X and Y. With it's Mega Evolution, it's Defense is well over 100, which is great for such a weak Pokemon. Most of the unevolved Gen V Pokemon are quite strong as it is (compared to other unevolved Pokemon, anyway)
Also, they may have given out Mega Evolutions to give out some unique types for new battle strategies. For example, an Electric/Dragon has not been released before, same with a Water/Dark. (To my knowledge, anyway). My favourite is Fire/Dragon with Mega Charizard X, nullifying Water. :D
Gen V brought in some pretty unique types into the mix, as it is. To mix it up more would be rather redundant.
Also, what MrK said - for Pokemon that have been favoured a lot over the years can become so much stronger! :3
So yeah, that's why no Gen V got Megas.
Hope I helped. :)