Some wallpapers are made with vinyl, the most toxic plastic on the planet. Vinyl (PVC) requires more emissions than any other plastic to manufacture, is not widely recycled, and contains massive amounts of chlorine and dioxins that are released during incineration or in time in a landfill, contaminating the earth indefinitely.
Switching to eco wallpaper when an alternative exists is a small way to make a big difference for the planet. Keep in mind the PVC you use today will be incinerated or find its way into landfills or the environment in the future.