What is the primary reason? Many people would love to work on such a project and some people would fund them. Would you agree that fundamental physics does not preclude a functioning nuclear rocket? Where would one build, much less test, such a device as a nuclear-powered rocket? We can't even agree in the US where to bury inert (mostly) ceramic radioactive waste. A live, radioactive (low-level, but still radioactive) exhaust stream is not going to happen in the US in the current culture.