Every winter there are many reports of the Mountain Rescue teams being called out to walkers in difficulties in bad weather. some of climbers on sheer cliff faces, but the majority are hill walking in rugged terrain, and have been overtaken by unexpected storms and are soaked through and chilled. Very often they turn out to be wearing jeans and sneakers. Once these get wet through they take a long time to dry, and in windy conditions they draw heat away from the wearer so that they risk hypothermia.
Good winter wear is closely woven, windproof and shedding much of the water that falls on it. If it does get wet through it dries quickly from body heat.
Jeans fabric absorbs water and holds onto it (think how long jeans take to dry when washed) and they are not wind-proof. In dry still weather, they are fine, but avoid wearing them in wind and rain.