Open source software is computer code that anyone can see. Everyone
can contribute to its growth and evolution, and so open source software can meet
new goals faster than software that is not freely shared. However, governments,
corporations, and users can have security needs that open source software does
not serve. Software that can be changed is software that can be diverted from
its original use. That is why governments generally do not use it.
I think that this is unfortunate, but the current case is that open source is not superior to proprietary software... yet. There are exceptions, of course, such as Firefox, but for the most part I think that the government should keep using supported software rather than relying on something that could go unsupported at any time.