Why do modern mountain bike forks no longer use boots on fork stanchions? Older mountain bikes used to have them on the forks, and lefty and headshocks still do. Why have these fallen out of fashion and would putting aftermarket fork boots on your stanchions be productive or not?
As technology goes, seal and wiper advance to the point that boots on fork stanchions becomes obsolete, for both economical and practical reasons.
The seal has advanced to the point that air suspension fork was possible and economical, leaving alone keeping dirt and grimes off the suspension.
So the answer is Technical obsolescence
Edit: And as @DeletedUser points out in a comment, the boots could cause more harm than good by holding moisture and dirt against the forks in places where they aren’t visible to the user.
Personally I think the trend to no boots is a bad one. If you look at auto shocks, which have been around much longer than mountain bike shocks, almost all of them have boots. I just bought a new car and the shock has boots. Modern mountain bike shocks without boots need to be serviced much more often due to dirt ruining the seals faster in my experience. So, I believe the removal of the boot guarantees the manufacturer and bike shop more revenue to service the shocks more often.