I think people often just eat what fits in with their lives. If you have a relatively sedentary life, you don’t need all those carbs. I think British dinnertime is getting later too - instead of 5pm, many people are eating dinner at 7-8pm+ now, and don’t want stodge before bed.
Wanging a stir-fry together or chucking some pasta on is quick, easy, very customisable to tastes, and you get your veg in. Similar with curries, you can even make a day/s in advance.
We can get ingredients now we couldn’t before, we’re aware of cooking styles now that we weren’t before - so why wouldn’t we take the best bits, that fit in best with our lives?
I don’t think it’s a new thing, but then my dear old mum used to cook us all sorts, some really bizarre things, including fish curry so hot it made you sweat, lasagne pie, and chilli served with baguette.