iPhone 5c says 10.3.3 is latest update?

Tried every suggestion I can find. Is it possible that a 5c will not update farther than 10.3.3? I have erased the phone and restarted to be sure it has plenty of memory. I have tried to update through itunes (via cord) as well. Both the phone and itunes say 10.3.3 is the latest update. Can this be true? Why can my 6 get more recent updates?

[Re-Titled by Moderator]