Not sure it’s marketing- younger DH riders tend to be slavishly led by world cup racing and DVDs, and those guys aren’t usually seen wearing this stuff. It’s soft knee pads these days.
I reckon it’s mainly just availability and quality- good armour is easily available, that wasn’t always true, it’s less of a drawback than it used to be. Also pragmatism- time off with injury is a pain in the bum. If your older it takes time to recover and you might lose money, if you’re younger it’s just time off the bike.
I’ve seen people out wearing leatts and full faces but no elbow or leg protection which seems a bit weird to me. Sure it’s worse to injure your spine than your knees, but which is more likely?