This might be a bit of an off-the-wall question, but I've always sort of wondered why old ads (like this candy ad from 1925) sometimes feature multiple paragraphs of text, which you'd never see in a modern ad. Were we just bad at advertising back then, or maybe is related to changing media / media consumption? Or something else?
Thanks!
Having briefly been a PR practitioner I might be able to point to a few things, some of which might challenge the basis of your question, some of which will help answer it.
A little bit. Yes. Or at the very least it was less sophisticated. The first person to really go out and analyze what was effective in advertising was George Gallup who worked for Young and Rubicam Advertising in the 1930s. Gallup would go on to be famous for the Gallup opinion polls which still bear his name but in the 1930s he really started to do some of the earliest research for what worked in marketing.
That being said some of the most powerful men in advertising in the mid century who had access to that research reached the opposite conclusion that you have here. David Ogilvy in Ogilvy on Advertising noted that, for instance, that "editorial" style ads were among the most effective. That long copy appeared to be more effective than short copy.
2 . I'd challenge your assumption here that this sort of thing isn't done any more. The "advertorial" is a persistently popular form of advertisement. The OED lists the first use of the term as going back to 1914. It remains popular today.
3 . Shifting media is important, but so is the nature of the marketing/advertising industry. The first full-service PR agencies arrived after the first ad agencies, and they are still a misunderstood beast. PR in the popular imagination is sometimes thought of as crisis management in preserving public opinion, or some sort of "spin-doctoring" about a controversial subject a la Thank You for Smoking.
PR also includes quite a lot of consumer PR, which might be extremely simplified into describing the art of taking a company's news, framing it in a positive light in the form of a press release and then getting that reprinted as actual news in the media.
The result is that advertising has in a way been "freed" to separate some of that audio-visual stuff from the longer form stories.
If Oh Henry! were doing this today instead of hiring an ad agency to write about how "Chicago women" discovered sliced Oh Henry! they might hire a PR agency to set up an actual event at a Chicago food fair where young Chicago foodies could try sliced Oh Henry! candies. The PR agency, in turn, would give selected writers privileged access to the event to write about and produce a press release about the food fair and how much everyone loved sliced Oh Henry! which would be sent to a broader range of media. That event could then feature in broader advertising.
The Pepsi Challenge is a good example of that process where real event-->media coverage-->advertising copy.
Source wise I think your best bet would be to read some of the classics on this like the books of David Ogilvy, or Luke Sullivan's "Hey Whipple, Squeeze This".