Why is that? I used to do mountain biking and I loved the challenge of climbing a large hill more than the adrenaline of descending it, which I found a bit too risky to my liking :D ...
Downhill issues have gotten a ton of attention, because there were problems to solve. Suspensions and geometry are making bikes a lot more stable feeling on descent. Try renting a 29er with a longer wheel base and a six inch suspension, and go down one of those previously risky feeling hills, you'll be surprised how much less risky it seems now.
Uphill issues have gotten relatively less attention, because, perhaps, a climb is a climb. There's little that can be done to improve climbs. Except suspensions are pretty good at firming up on the fly. And dropper post seats are amazing at being able to switch from descent to climb instantly.
In other words, climbing on today's mountain bikes is better - slightly better - than it used to be. Going downhill today is much better than before.
Also, in my opinion, the tailgate pad has made it practical to run [downhill] shuttles using pickups. Prior to that, you had to get a roof rack + 4 trays = $600+. A Dakine pad is $125. Without the shuttle, you have to climb.
I think it's a bit more complicated too: they liked to go downhill, but they needed to get back up, and wanted to not have to maintain the bike after one trip down (repack), so bikes that could climb were desirable.
Ah! Excellent example.
After we realized we didn't need 30 gears, new suspension designs also became possible.
You're right that rear mechs frequently die though.
I'm pretty sure the derailleur clearance is actually lower on the old 26" bike! The geometry of the derailleur has changed, and when it's on the biggest rear sprocket, the old Nomad derailleur's center of mass is below, lower than the large sprocket. On the new 29er, the derailleur is mostly above the bottom of the largest sprocket. It seems like derailleurs have improved in order to allow the larger sprockets that single front chainrings call for.
The idler (lower pulley) on the 29" is almost certainly further from the axle than on the 26", however the larger wheel means that clearance from the ground is about identical.
I don't think I've ever hit my derailleur on either bike for lack of clearance -- even though I have caught my pedal on a rock or branch multiple times. I have hit the derailleur during a crash, but mainly because the bike was flying around and smacking into things. I'm guessing because the derailleur is behind and inside the path of a pedaling foot, it's mostly protected.
As someone who has been mountain biking for over 25 years I would say the statement that uphill has been de-emphasized over downhill is absolutely correct. Enduro racing (where the uphills aren't even timed) is a nice example of that.