What often happens behind the scenes is that requirements are written to a specific software product that is already effectively pre-determined. Like let's say it is a complicated inventory tracking system, out of 50 or so requirement points, every one basically matching the system that L3 provides without naming L3. Stuff like that.
It is vital to do behind the scenes networking, hiring insiders, that needs to happen besides also having someone full time that knows how to jump through the "official" red tape.
I don't think the government means to lock out small vendors. It's just that bureaucracy tends to generate bloated and inefficient systems like this, in which the only part of the contract the vendor can afford to skimp on is the implementation itself (and not the lawyers, negotiators, managers, etc.) The people who work for the government want good, working systems, they're just prevented from choosing vendors who can produce them because using common sense is not sufficiently bureaucratic.