Referral fees would be an easy explanation. Another reason is that SaaS companies, particularly in the enterprise space, need to reassure their customers about the quality of their infrastructure, reliability and redundancy. So if the servers are hosted in Equinix instead of under a developer's desk, it conveys a level of security and competence that the company's brand should benefit from. I have never seen a Rackspace facility, so I don't actually know what they are like, but the Equinix datacenters I have been to are better than any major corporate setup I've seen.