My thought is, urban rails have their origin in skateboarding, so sometime or another, snowboarders realized they could have fun on urban rails with their snowboards. They liked Urban rails so much, they wanted to bring them onto the mountain, so they could hit them up on a regular day as well as urban missions. Of course, many skiers are also skateboarders and would also have realized teh potential of rails to be a good time sliding on skis. When rails started showing up in 'snowboard parks' of course skiers would hit them too.