That’s usually because some dependencies of dependencies broke in the meantime - something that inevitable happens when trying to install an outdated version (Indico 2.0), which is exactly why we really encouraged people to migrate from 1.2 shortly after 2.0 was released! 
Anyway, I’m afraid I cannot reproduce your problem. I just created a new virtualenv with Python 2.7, and after installing the old pyatom version (using the command here) I was able to pip install 'indico<2.1' and then pip install indico-migrate just fine…
My guess would be that, since you are on Python 2.7.5 (the ancient version coming with CentOS…), your pip and/or setuptools are ancient as well. Upgrade them using pip install -U pip setuptools and then try installing indico and indico-migrate again. Old versions of these tools do not know how to filter out packages not compatible with the current Python version.