Snowboards are able to glide because of viscous friction. When you're shredding the slope, the underside of your board heats up because of the friction between board and snow. This heat melts the snow your plowing and results in a very thin film of water between the board and snow, ergo the sliding effect. This thin film of water is a double edged sword. On one had, it's what allows you to full send the slopes. On the other hand, it is responsible for slowing you down.
Capillary drag is one of the phenomenon that steals your send. The surface tension of water makes it cling to the bottom of your board and stretch, this pulls your board back and reduces your speed. The other thief is known as triboelectric drag. Your board develops a static charge which then attracts any contaminants on the slope, dust, pollen, etc. This gradually builds a layer of dirt on the underside of your board, the new gritty surface creates more drag and abrasion.