"You need to also enable the 'optional' repository to use EPEL packages as they depend on packages in that repository. This can be done by enabling the RHEL optional subchannel for RHN-Classic. For certificate-based subscriptions see Red Hat Subscription Management Guide. For EPEL 7, in addition to the 'optional' repository (rhel-7-server-optional-rpms), you also need to enable the 'extras' repository (rhel-7-server-extras-rpms)."
Add extra instructions for Red Hat 7 (not needed on CentOS):
From https://fedoraproject.org/wiki/EPEL
"You need to also enable the 'optional' repository to use EPEL packages as they depend on packages in that repository. This can be done by enabling the RHEL optional subchannel for RHN-Classic. For certificate-based subscriptions see Red Hat Subscription Management Guide. For EPEL 7, in addition to the 'optional' repository (rhel-7-server-optional-rpms), you also need to enable the 'extras' repository (rhel-7-server-extras-rpms)."