Probably the easiest way to answer this is by analogy. Remember back to the Betamax/VHS wars? How about CD/Digital Tape? Blu-Ray/HD DVD? The winner in each category had at least two of three benefits. First, lower cost. Second, media availability which is why anyone bought a device anyway. Third, ubiquity, availability in multiple formats. Believe it or not, technical superiority was not a driving factor. Beta was better than VHS, and HD-DVD better than Blu-ray. These three elements taken together drove market share which in turn determined who won the war and as such became the de facto standard.
So we proved that people like standards. Being the defacto standard meant being cheaper, having more media available, and being available in more form factors. Which in turn led to even cheaper devices... see the effect? What's amazing is that rather than fighting it out in the market to create a defacto standard, a standard can be set up front and thus drive the development of better solutions. (NOTE: open standards are simply standards for which there is no licensing fee. For example to use RAMBUS or JPG there is a licensing fee required and hence there's been a serious withdrawal from using either as a standard).
The goal of open standards is to drive interoperability, from day one, to maximize flexibility while keeping costs down. In turn the interoperability combines the efforts of all market entrants to achieve the three goals of driving down costs, increasing options, and enabling the development of new form factors. In fact open standards are everywhere. Your car has OBD-II, On Board Diagnostics 2nd Generation, which every automaker must comply with so a mechanic can connect a device and get a little help from your car in diagnosing a problem. HTML is another great example and there are plenty of others from email (SMTP, POP, IMAP) to managing IT (ITIL v3). In fact many of the technologies underlying cloud computing are themselves based on open standards. If we want cloud computing to be as ubiquitous and consumable as we all say, then we need open standards upon which to base interoperability. An isolated cloud is, in essence, nothing more than a large silo. Clouds need to have virtual boundaries which can be traversed easily and quickly as needs dictate.
So the real question is why aren't there open cloud interoperability standards?
The short answer is there are and more are coming. Standards evolve over time via learning. The original answer was the advanced advocates of cloud want to recoup their investment and make fat profits which is more easily done with proprietary technologies. However they almost all come around eventually. Several technology companies are very open to open standards including Rackspace, CA, AT&T, IBM, and HP. Others are coming late to the game like Amazon, VMWare, and Microsoft and still others may never join the team such as Google and Salesforce.com/Force.com.
Open cloud standards are here today in one form or another as evidenced by the Rackspace OpenStack and Eucalyptus. In fact OpenStack is quickly gaining momentum and warrants further investigation if you were previously unaware. In addition several governance groups have appeared beyond the original Open Cloud Manifesto, such as the OMG Cloud Standards Group, each working to establish open standards.
So when looking at cloud technologies keep in mind that proprietary solutions move you in the wrong direction. The days of proprietary vendor lock-in are not over. Open standards are out there so make sure they're part of your architecture and part of your technology selection criteria.