It shouldn't come as too much of a surprise. At a time in history when we have pretty much unlimited access to all the world's culture and history people are becoming less and less curious, preferring to spend their time in curated echo chambers.
I'm 52. When I was a kid (here in the UK) there was very limited kids' TV. Even in the long summer holidays, kids' stuff was limited to the morning. So if I found myself allowed to watch TV on a rainy afternoon it was mostly old B&W movies I was watching. There were far fewer books aimed at kids or teenagers, so I went through my mum's bookcases and read everything from Dickens to Len Deighton (how many parents even have bookcases containing anything other than cookbooks and celebrity memoirs now?). So I imbibed from a huge range of culture that spanned decades at least, if not centuries - as did a lot of my peers.
Compare to now, where children have multiple channels devoted to giving them exactly what they want, as a means of advertising to them - no need ever to watch anything outside the comfort zone. Countless 'child-friendly' books - all variations on the same themes - all easy to digest; no need to explore the library or pick a book of the bookshelf to counter rainy-day boredom.
Dumbing down is a fashionable phrase, but that doesn't make it any less real. A dumbed down audience is a lot easier to sell to and to make money out of so, as I say, it really shouldn't come as too much of a surprise.
BTW, Trump isn't a solution to our dumbed down culture - he's a product of it.