From what I've read, GPS satellites have been always put in orbit one at a time. In contrast, Galileo (aboard Ariane or Soyuz) and GLONASS (aboard Proton) sometimes are launched at the same time. Is there a clear reason GPS doesn't follow the same approach? It seems a lot cheaper. I think it's important most of the time consecutive GPS launches don't aim for the same orbital plane, but this was not the case for SVN-31 and SVN-37. They both ended on plane C within a two month difference.
Some guesses: Delta or Atlas rockets don't allow multiple payloads? They don't think it's worth risking several satellites on a single launch? Really tight schedules?