Dense seating in economy, is the sole reason. The airlines decided to ignore the Premium trend, and made economy basic as densely packed as they can, and provide limited premium economy to guarantee it had high margin and was full. So, business and premium on a 787 is great, but economy is really really tragically bad, as a long-haul experience.
787 for shorthaul, nobody cares. Ask anyone who sits in the cheap class about 787 for >10hrs, compared to the 747 economy, or the 380 economy experience.
The lower effective air cabin flight height (higher pressure, moister air) and the new lighting, and bigger windows are very good for everyone, But the shrinking seat, shrinking legroom, lack of aisle access, the shrinking toilet? very bad.
I fly premium and business. I am empathising what I am told, not what I live. I've flown 15h+ flights up front in a 787 and it was fantastic. Some of the best flights I've had over trans polar landscapes. Amazing. Sucks to be poor.