It's a little of everything and more when it comes to the comic covers.  It definitely comes down to the sign of the times. Back before computers and Previews, comics had to stand alone to catch the eye. Stan Lee always called this the 'Wow Factor'. When the reader goes into their local A&P or 7-11 or newsstand, that cover has to tell the reader everything and then some in less than a blink of an eye. From that glance, the book is checked out.  It was simple marketing for a simpler era.  But it helped pushed sales big time. 

With today's technology and the advent of Previews, pretty much everything is told to us in 2 months advanced notices.  My problem is today's comic covers aren't even covers; they're now just future prints for the artist to sell at his/her booths at convention.  They  tell absolutely NOTHING of what the interiors are.  There's no Wow Factor these days.  There's "Hey, that's a good looking cover," but not still tells you nothing. 

There's some brilliant cover artists in today's market and have a following of people that buy their covers just for them and not their interiors;   I happen to be one of those people for a couple of artists.  They have good sense of design but as I said, its built for a collector's market... almost like hanging a poster/print up on the wall.