I haven't seen this posted here, but it's a translation of an interview, which gives a bit of background about why some Mega Evolutions were made, and why some weren't
A lot of interesting stuff in my opinion. I like that they said that the process of creating a Mega can start in different ways, sometimes the design is the main priority, other times it is the stats.
Onto specific ones, that the main reason that Mega Slowbro was made instead of Mega Slowking was that it wouldn't work that well with the concept of the shell swallowing it, which apparently was the basic idea. Mega Metagross gaining more speed instead of attack, as well as not getting Levitate to make sure it was versatile enough. Mega Pidgeot is also interesting, since it seems to be made because it is the ace of Blue making it "iconic"
foreshadowing?.
They also mentioned that they wanted to make more different Mega Evolutions and not too many alike, which is why no other Normal/Flying type has a Mega yet. Mega Salamence was more of a simple case of "it looks strong, so it has to be strong".
It's also a bit relieving that they said that they will reject a Mega Evolution candidate if the basic design feels complete, although I do question that in some situations.