Considering the facts that are known (e.g.: http://www.digitalbond.com/2012/01/31/langners-stuxnet-deep-...) it is nigh impossible for Stuxnet to be commissioned by a nongovernmental entity.

The sheer complexity is off the charts. Stuxnet's sophistication and complexity is an cybernetic equivalent of Manhattan project. E.g.: As a domain expert on industrial automation of this kind Langners states that whoever created Stuxnet _had to have a testing facility_. How many hackers do you know who build uranium enrichment centrifuges to test their cyber attack tools?

Yeah thought so.

Edit: TL;DR: There are two pieces of evidence. No.1: Motive, No.2: Sophistication and complexity.