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.
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 ...