Certainly it is possible to develop capable IFS. Most Baja racers use IFS. Most side-by-sides use IFS. But instead of getting something truly capable, we get something like the Land Rover Defender. Popular production 4WD IFS vehicles designed for road use (where IFS is clearly better) do not have the suspension travel necessary to compete off-road with the solid axels on the Wrangler.
From what I saw, the Wranglers still had 4 tires on the ground while the Bronco had only 3.
Flex isn't the limiting factor to an independent suspension, weight transfer is. In fact, a solid axle rig with less flex can be more stable on uneven terrain than a rig with an independent suspension.
When a solid axle articulates (one side stuffed, the other drooped) weight is transferred from side to side as well as fore and aft, particularly when the rig is crossed up. This helps to keep the chassis level. When a tire droops into a hole, weight is transferred to the side that's stuffed.
When one side of an independent suspension falls into a hole, it doesn't droop and the other side doesn't stuff. Maybe a little but the tire falling into the hole transfers little to no weight to the other side. The suspension doesn't articulate well when a tire loses contact with the ground. The result is the chassis falls into the hole instead of the suspension. The weight of the vehicle transfers to that corner more and more as the angle increases until it reaches its tipping point.
Under the same circumstances, a solid axle suspensions transfers the vehicle away from that corner. Of course, it can only transfer so much before it also reaches its tipping point and has a wider margin of error as the tipping point is approached.
Note: There are other factors used to explain what's going on other than CG, such as static CG, dynamic CG, roll center etc., but I'm not smart enough to try to include them.
This doesn't mean I think only a solid axle suspension is worthy for off-roading. Both suspension types have their strengths and limitations.