Lets say you have the best "mobile app" to do whatever, XYZ.
Oh great, govt customer wants it. They see it today. Well, thanks to procurement, it will be 18 months before they actually buy it. But, by that point, the product will have changed a good bit - well because that is what software does now a day. But, that isn't what Gov manager wanted - they want what they saw 18 months ago.
The sales cycle will chew startups up and spit them out. Not many can accommodate 18 month+ sales cycles.
And that even excludes the issues of continuing resolutions means they don't have cash to buy buy many shiny new objects.