There are a bunch of reasons why panniers are not favored for bikepacking. For one, the entire premise behind bikepacking is two-fold. First, to be able to ride your bike with as much fun and enthusiasm as you do every day. Secondly, to do that on terrain that is fun to ride, mostly singletrack. Touring, as traditionally defined with panniers, is almost just an exercise in hauling all of your crap from A to B with little regard for how fun and spirited the ride is.
Bikepacking bags (rack less) keep the weight balanced across the entire bike. They keep the bike profile slim, and in most cases, there's usually way less than 15 pounds of gear on the bike. In other words, you can rail turns, launch over water bars, and generally ride your mountain bike like you want to ride your mountain bike. Hit a jump with panniers and you'll leave behind a crater, filled with busted panniers.
Bikepacking on singletrack also requires a good deal of hike-a-bike. Trail builders didn't always think of us poor sods on bikes. Pushing a bike around panniers is a pain in the butt. Plus, panniers leverage your gear off the sides of the bikes, which feels odd. Lastly, I don't care how good your racks and panniers are, they rattle around and in true offroad settings, just do not hold up. I've ridden thousands of miles with panniers on pavement with no problem only to have the same panniers fall to pieces on a real offroad romp.
I think the primary reason is that bikepacking is all about being as light as possible. Not just pretty light, or even ultra-light. I'm talking hyper-light. As such, few would even need the space of two small rear panniers.