DreamHost VPS is a managed VPS, which means that there are constantly automated processes that upgrade and configure the virtual machine for security and regular maintenance. This allows users never to have to worry about 0day security issues with Apache or nginx, for example, because DreamHost does the upgrades for you. Mixing robots doing the maintenance and humans (via root access) proved to create more troubles than it’s worth.
If you need root access, I suggest you to try DreamHost cloud on https://cloud.dreamhost.com: it allows you a lot more freedom and power than our VPS and it can also be quite cheaper. R will run great on DreamHost Cloud resources, on a more modern Linux distribution, too.