One fairly major reason is the economics of the nuclear fuel producers (who also tend to be the people building the power plants). Simply put, enriched uranium fuel pellets require huge resources to produce, and are !&^" expensive. Moving to Thorium would remove a load of their revenue stream, as it is a much simpler fuel to produce (well, for the liquid fuel reactors anyway).
Not quite so simple. The nuclear industry wanted/needed federal dollars, so not being in the uranium game looked like a bad bet. Once you have all that infrastructure invested in uranium, investing additional resources into thorium looks less attractive.
The department primarily responsible for the research of nuclear technology including nuclear weapons development is the Department of Energy. Therefore the preferences of the DoD influence DoE decisions.
