Why are breeder reactors not more prevalent? I recently learned about breeder reactors, which are able to create more fissile material than they consume.  They seem to be superior to the types of nuclear reactors currently in use.  What are the reasons why breeder reactors are not more prevalent.
 A: dmckee has the idea.  The reason we do not use breeder reactors in the US is politics in general and nonproliferation specifically.  Jimmy Carter essentially pulled the plug in 1977:

In an April 7 press statement, President Carter announced, “We will defer
  indefinitely the commercial reprocessing and recycling of plutonium produced in the U.S.
  nuclear power programs.”9 He went on to say, “The plant at Barnwell, South Carolina,
  will receive neither federal encouragement nor funding for its completion as a
  reprocessing facility.” (It was actually Carter’s veto of S. 1811, the ERDA Authorization
  Act of 1978, that prevented the legislative authorization necessary for constructing a
  breeder reactor and a reprocessing facility.)

Nuclear power is perhaps a uniquely political source of energy. It is simply not commercially viable without government support.  The thing with breeder reactors is that you can't just take the fuel out of one and pop it into another reactor, you have to dissolve the spent fuel in acid and "polish" it to make MOX fuel.
At the Savannah River Site in South Carolina there is a project under way to build a facility that will make MOX fuel out of dispositionable nuclear weapons material (it's where I work, in fact).  The feed material, though used in weapons, was indeed produced in breeder reactors.  The technology behind the practical implementation of breeder reactors and the production of nuclear weapons is clearly linked.
You might also be interested in my answer to a similar question on the use of thorium reactors.
A: Simple answer, but not physics.
One (kinda, long and compound) word: nonproliferation.
Uranium bombs are harder to make than plutonium ones, so breeders make it easier to build bombs.
A: Uranium is also substantially cheaper than plutonium and the difference between costs saved and the adoption cost is not enough to warrant adoption on a large scale.
Another point is that, for many of Soviet-era breeder reactors, the coolant was liquid sodium, which, as we all know, doesn't react in a safe manner with water. Add a leak, and you've got yourself a safety problem. Water cooled reactors are more costly, and it would make no economic sense to adopt either of them. 
Development of safe and efficient reactors was hindered by hydroelectricity in the USSR, due to safety features rendering nuclear energy less cost-effective than hydro. This is the root cause for the major part of the safety failures of Soviet nuclear reactors (and Chernobyl). 
I can't really comment on the lack of US development to breeder reactors, as they don't come close to my area of expertise, but I would likely point to enviromentalists' political pressure, and reduced Uranium costs from Canadian imports.
