Why is Conan The Barbarian (2011) rated R?

Probably because of the blood and gore that would accompany the violence and strong sexual references. I imagine it as much like Spartacus: Blood and Sand...

DId they remove Conan the Barbarian (2011) from theatres?

It is no longer being shown in any of the theaters here in the USA (central Ohio) any longer either. Shows a DVD release date of November 21, 2011, so you will be able to watch it soon on DVD.