There are two main factors.
First of all, there is the simplification of project management, because the government would be dealing with one shipyard, rather than dealing with two partners that you have to monitor.
Then, the aspect that is probably the most important is the economies of scale generated. There is always a learning curve when doing something for the first time, and this applies to the first ship that is built. These economies of scale are realized by a shipyard and can allow the second ship to be built more efficiently, and therefore at lower cost. This is not the case when two yards both build a first ship.