Per usual, the UCI is banning something that doesn’t make any sense.  From the new rules posting:
Protections in Downhill/4X:
An approved full-face helmet must be worn properly fastened both when racing and when training on the course. The helmet must be fitted with a visor. Open-face helmets may not be worn.The UCI strongly recommends that riders wear the following protection:
– back, elbow, knee and shoulder protectors made of rigid materials;
– protection for the nape of the neck and the cervical vertebrae;
– padding on shins and thighs;
– full-length trousers made from rip-resistant material incorporating protection for the knees and calves, or broad-cut shorts made from rip-resistant material plus knee and calf protectors with a rigid surface;
– long sleeved shirt;
– full finger gloves.Clothing in Downhill/4X:
All tight-fitting clothing is not permitted
But, here’s the reasoning:  Marketing types don’t like the skinsuit because it doesn’t help sell bikes, clothing or gear.  So what’s to become of us XC folk…do we need to start buying end of season blowouts on baggy shorts and jerseys so we won’t be caught with our (tight) pants down if they change the rules on us, too?