Things are different for agencies tied to defense or intelligence gathering. I would imagine the NSA tries to do as much in-house as they can. NASA design secrets leaking aren't nearly as potentially damaging as internal NSA plans or techniques leaking.

Of course, Snowden was a contractor so that shows they do allow contractors with TS clearances to work on their projects, but 1) it's usually only specific firms that they trust and 2) the contractors may help build the systems but probably will be told little or nothing about the exact end usage and results.