Because despite having insane amount of energy in Uranium, you can't tap into it. Uranium melts at 1000 degrees centigrate, so thats your rough upper limit for

Because despite having insane amount of energy in Uranium, you can't tap into it. Uranium melts at 1000 degrees centigrate, so thats your rough upper limit for how much your reactor can heat hydrogen. If your Uranium melts, it will be leaving the rocket with the hydrogen and spreading radiation everywhere. Basically like Orion but worse. A normal chemical rocket reaches higher temperatures. A nuclear bomb will heat anything to millions of degrees. Much more efficient, and any exhaust is moving so fast it will escape the solar system without bothering anyone, provided its pointed the right way.