Blaziken was likely showcased because of all the Starters it probably lent itself best to being in the show case of Megaforms.
Notice how 5 of the 6 Mega's revealed were Bipeds/Humanoids
It's much easier to come up with a powered up form for human shaped Pokemon that stem from their base shapes and designs.
They'll likely trying to ease people into Mega Forms with what they assume are some of the cooler/better designs.
Gen 1 Starters are the originals, so there Mega Forms HAVE to be show cased together.
A Gen 2 and Gen 5 starter wasn't chosen because they're too bland in terms of design and even their Megaforms won't likely wow the kiddies.
That leaves Gen 3and Gen 4, from which they can choose from, so they probably picked what they thought was the coolest design of those 6.
This does not bode well for those other 5 though then if Blaziken was considered the best.
