Effectively, this is because an advertisement's chance for success increases as the number of pertinent merchandise facts included in the advertisement increases. You reel them in with the headline and the picture, and then the body copy sells the product. Long copy assumes that you've got a half-sold prospect, with little to no time however, so you pack in your promise in there.
The caveat is that if you want your long copy to be read, you must write it well. You won't hold many readers with those mushy statements of the obvious and vague promises. (This is it...You step back and think...) The worst sin here is that the copy has no contrast, no one wants to read white text on a light background. It's just tiring on the eyes.
It's not that long copy isn't back (it always has and always will be), but that Apple really executed this terribly.
I know marketing types will agonize over each line like poets imagining dramatic public approval, but most people will only see the words "Made by Apple in California", which brings about a "well, duh" moment, at least to me.
Here are classic examples: http://robertrosenthal.typepad.com/.a/6a00d8341c511c53ef0120...
http://www.perrymarshall.com/images/micro_tv_breakthrough.jp...
http://copyquickstart.com/call/sugarman/homework/home_comput...
Sorry that the last one has just one of two-paged spread. All I got via Google.
EDIT to add: Oh, look at this, from the JS of JS&A fame:
http://succeedinyourpajamas.com/wp-content/uploads/2013/06/J...
This was more of a meta-mini-rant about how we have these tools available to efficiently communicate meta-information along without main message, but especially news media outright refuses to make use of it.