For us the primary reason is because not only is Python 2 the default in RHEL 6 and RHEL 7 (and thus downstreams like CentOS and Scientific Linux), they don't e

For us the primary reason is because not only is Python 2 the default in RHEL 6 and RHEL 7 (and thus downstreams like CentOS and Scientific Linux), they don't even ship with Python 3 or have it in the base yum repos, so getting it installed on our thousands of diverse systems is nontrivial. Furthermore we have a large base of scripts, owned by many people, that use Python as they would use bash, so changing the default Python on them will be like linking /bin/bash and /bin/sh to /bin/csh.