I think it is unlikely for most legendaries to get megas. I'm guessing it will be very rarely that we will see a mega-legend, with mewtwo being one of the few exceptions because of either "muh pandurring" or "scientists did it", depending on who you ask. The Latis getting megas is likely because soul dew fits in with mega stone creation, and also to raise hype for ORAS.
Also, going with what
>>19263017 said, who really gives a shit about the birds. Mewtwo is one of the most popular pokemon ever, probably the most popular and well known of any legend. Not many current fans like them anywhere near as much as mewtwo, and even fewer former fans like or even remember them.
tfw one of my favourite legendaries is unloved 
