This isn't going to change until there is a thought process shift for leaders on both the government and contractor side. I worked on a project that fought tooth and nail to create a project using an agile development process and it was one of the best projects I worked on for the government. It was killed due to politics, but the feedback, functionality, UX, and collaboration up to that point were great. Everything else I did was waterfall and we always has the same cycle.

  while (true) {
    // Contractor working for a year
    Government: This isn't what we wanted!
    Contractor: We met all of the requirements... See all of the boxes are checked.
    Government: Well we want to change 1,2...n things.
    Contractor: Okay, Let's do a follow on contract.
    Government: Okay, Here is the money; Go.
  }