The hardest part of any Mars flight is simply getting from Earth's surface to LEO. That step literally costs more delta-v than the rest of the mission combined,

The hardest part of any Mars flight is simply getting from Earth's surface to LEO. That step literally costs more delta-v than the rest of the mission combined, and nuclear rockets really aren't up to that challenge - NTRs don't have the TWR, and I don't really think Orion needs explaining on that front. The other thing is that we can get to Mars with chemical rockets. Using an NTR wouldn't make the journey meaningfully faster - we'd just bring a larger payload - and while I'm sure that almost everyone would rather triple engine Isp, personally I'd rather just go to Mars already instead of waiting for another couple decades. If we do start spreading out into space, the vast majority of our early trips will be to the moon or to Mars. Don't overengineer the problem - an cheap and adequate rocket now is better than an excellent rocket later.