Purchase of commercial-off-the-shelf items is rarely purchased through competitive bids. Major vendors of computers, software, furniture, and I would expect cameras, lenses, etc, usually have a negotiated schedule of government-only pricing that any government agency can purchase from. If the vendor doesn't have their own schedule/catalog, there is usually a 3rd party intermediate seller with a schedule. These schedules are usually compiled and managed by the General Services Administration (thus the term "GSA Schedule"). Any government agency can purchase from these schedule items. As long as the purchase is made from pre-approved GSA schedule vendor, there is no requirement to obtain competitive bids. The rationale is that in assembling the schedule of vendors and prices, the GSA has already conducted a competitive selection process. Therefore the purchasing agency doesn't have to run a separate competition.
For example, B&H Photo is a vendor on a GSA contract.http://www.bhphotovideo.com/find/federal-gsa-contracts-gov-corporate-edu-sales.jsp
To purchase from a GSA-approved schedule, all the agency has to do is get approval of the expenditure through their own internal purchasing office. If the requestor's organization has money in the budget, and a local manager with purchasing authority agrees to sign off, the purchase is made. End of story.
I'm in IT and my agency makes purchases for hundreds of thousands of dollars in purchases for IT equipment and software every year without having to go through a competitive bid process because the GSA-approved vendors and pricing have already been compiled by the GSA. We just look through the catalogs, find what we need, and make the buy.
When I was in contractor/industry side, I spent quite a number of months working on bidding on NASA business so I know NASA also has its own pre-negotiated Enterprise-wide equipment catalogs and schedules with industry vendors.
Most likely someone in NASA determined that, for whatever reason, Nikon more closely met the technical requirements for NASA's mission. So it would just be matter of going to the catalog and making the buy.