Because have a standard that applies across all supplies is inconvenient as it prevents lock-in.
This is all about a single supplier becoming an incumbent and snaffling a nice pile of government lucre. This looks to the be same as the JEDI contract where Amazon promptly starting ranting "It's not fair" when it was awarded to Microsoft.
Unfortunately once contracts get to these sorts of sizes there are so few with the capability to deliver that unless there is enforced cooperation (never going to happen) there is always going to be someone who is unhappy.
Maybe NASA should go back to doing it themselves.........
Good luck with that.