Why the exponent is a power of 2 in Ring-LWE? I was reading some papers on Ring-LWE. I found almost all of them talk to choose the polynomial modulo $x^n+1$ where $n$ is a power of $2$. I did not understand why this condition is necessary?
As @Raoul722 pointed out the polynomial should be irreducible.

One should also add the fact that in the original Ring-LWE article security proofs hold by sampling the error with a spherical distribution and the poly $f=x^{2^n}+1$.

In How (Not) to Instantiate Ring-LWE, from Invulnerability instantiations subsection it's specified that you could in fact select any number field and still be a hard problem but then you have to change the error distribution. 

To answer your question, the reason is just to make proofs easier.