Are Slavic Russians the descendants of the Nordic Rus?

To some extent yes. There is reasonably compelling evidence for the theory that the Ruric Dynasty of the Kievan Rus was founded by people from the region around Uppsala or the Southern coast of present day Finland. If one chooses (rather arbitrarily perhaps) to begin the history of present day Russia there then they are descendants of the vikings.

However, the Ruric dynasty was never large in numerical terms, it was in all likelyhood a small warrior elite that ruled the lands, never a majority population. They seem to have had a very small impact on linguistics and culture. The Vikings in History by F. Donald Logan states that "...in 839, the Rus' were Swedes. In 1043, the Rus' were Slavs."

If so- why aren't Russians considered Nordic?

Because "Nordic" is a cultural identity particular to Sweden, Denmark, Norway, Iceland and perhaps to some extent Finland. It is very distinctly different from the Russian or Slavic cultural identity and calling Russians Nordic makes no sense. The cultural identity of the Ruric dynasty was quickly absorbed into the greater Rus culture and gradually became less important as other cultures had a much stronger influence on it.

If you want further reading I suggest:
This thread with excellent responses from /u/facepoundr and /u/MrMarbles2000 regarding the influences on early Rus culture.
This thread by comrade /u/rusoved regarding the Russian cultural identity.
Or this thread by yours truly regarding the viking origins of the Ruric Dynasty of the Kievan Rus and Viking influences on Russian culture in general.