It’s a good question, here are a few thoughts. Tracking over rough terrain is much faster and easier on larger wheels. Modern MTB geometry has solved a lot of the maneuverability question, allowing wheel size to act as another form of suspension. And the sport and industry as a whole has been more focused on making more and more “capable” bikes, which means bikes that can eat up very rough terrain and remain stable, sacrificing some degree of maneuverability.
Smaller wheels are, as you say, lighter and more maneuverable, but it’s only one element of the sport. They get hung up easier in rocky terrain, they don’t cover ground as easily, they are more skittish in very steep terrain, and they transfer more impact to the rider. So for most riders, particularly if we’re riding bikes with modern geo, the trade offs aren’t worth it for smaller wheels. Reduced offset forks, shorter chain stays, steeper seat tubes, longer front ends, and slacker head tube angles have created bikes which ride far better over steep, rough terrain and retain an incredible amount of maneuverability.
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