Why does contract progress reset if you switch to a new one before the round ends?

I was doing the Loose Cannon contract and completed it halfway through a Banana Bay game. I thought, since I just need to wait until the round is done to turn it in, I could get started on a new contract as well. Only thing is, after the round has ended, my Loose Cannon contract has reset and I was back at zero points.

Why can't contracts be completed before a round ends? Seems kind of silly to have to wait for the round to end and not be allowed to do any other contract.