I arrived at CERN when the decisions about this were made already, so I'm mostly involved with the actual implementation. I would say that the main reason is that a lot of experiments (not just the higgs boson search) are going on here right now. Somehow you need to distribute resources and let people do stuff with them and the centrally managed Openstack/Puppet stack vastly facilitates the task. The fact these solutions are open source and we can add our stuff (or others) on top of the code is also quite important for us.