IT managers and experts are usually quite aware of this kind of thing, and in the early days of adoption, there's a very understandable reluctance to start using a technology and get locked in to a particular vendor. Vendor lock-in is especially dangerous when a technology is nascent, like cloud computing is now.
In such an environment, open source technologies are a much safer fit for early adopters. If, for whatever reason, a particular tool doesn't do the job, it is much easier to migrate to another tool from an open source application than a proprietary one. IT users know this, and this fact has put wind in the sails of open source technology in cloud computing.
* Structure. The final reason open source is so central to cloud computing is not market-driven, as the first two conclusions were. But it is a decisive advantage related to very structure of the open source ecosystem.
Open source vendors are leading here because open source itself enables developers to bring together disparate tech elements freely. A proprietary vendor, on the other hand, is forced to pull together and build everything themselves, which puts them at a serious disadvantage in terms of resources and licensing.
In fact, cloud computing is itself derived from deeply mature open source technologies. Amazon Web Services, the Emerald City in the Oz of cloud computing, is built on Xen and Linux. Rackspace's cloud services are also composed of open source technology.
Here, at least, proprietary technologies are the ones playing catch up with open source, and not the other way around. It would be two years after the launch of AWS, for example that VMware would launch its Vcloud Director.
It is these three reasons, these pillars, that are holding up cloud computing as the next wave of open source innovation. Without major marketing dollars and high-pressure sales, open source has fulfilled its promise: delivering high-value technology with speed and sophistication... and a non-astronomical price-point.