Because there is a requirement to maintain a certain amount of capability (so if a spy satellite fails, they need to have a spare to launch to replace it in short order), and the long lead times (say 5 to 10 years) to build these satellites, the DoD orders a number of satellites, and some of them may possibly never actually end up being flight vehicles.
These two satellite 'cores' are just such spares. They weren't launched because a new generation of spy satellites were put into operation before these were needed. There are probably no other examples of these things still in orbit, though the model was probably used for a decade or more.
Remember, NASA wasn't given two completed satellites, they were given two mirror assembly's and the associated bus and structure. It is up to NASA to design and build a useful science satellite with them.
As to the DoD having something better, they probably do, but it isn't the mirror that is better, it's that the bus and structure will be of a different design. Optically, the limitation on a downward looking spy satellite will be the atmosphere, not the mirror or other associated optical components.
Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/CpYVHkjYk68/story01.htm
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