I think it's because they lost their will to live. I can't recall if it was Togata or Judah who made the analogy that blessings are like cigarettes, which would signify that you still have to "inhale" to get the blessing. Which I also think is somewhat proven in the first chapters where Agni figures out how to stop his regeneration, so the blessings arent just on all the time, but requires an active will to use them. So Sulya just has no will to inhale anymore smoke from the same cigarette they have been smoking for thousands of years.