For super long trips, efficiency is king. Coupled with the ability to ability to burn continuously, solar-electric and nuclear-electric are way more efficient than nuclear-thermal. Ion engines may have low thrust -- just use a lot of them.
Nuclear-thermal's Achilles heel is that you can't use it to boost off Earth. So you need a high thrust chemical engine to get it into space. It's hard to make the numbers work -- nuclear-thermal may be more efficient than that chemical engine, but it's dead mass while you're boosting it out of the Earth's gravity well. If instead you can use the same chemical engine to get to Mars that you used to boost yourself out of Earth, you don't have that dead mass. If you want to get to Mars fast, just boost some extra fuel on an extra ship.
Nuclear-thermal looks nice on paper; it's probably the highest efficiency high thrust engine that's achievable with today's materials. But it's hard to find a good use case for it. A manned Earth->Mars run used to be it; but SpaceX's plan for in-space-refueling shortens the trip time just as much, if not more.
Nuclear-thermal's Achilles heel is that you can't use it to boost off Earth. So you need a high thrust chemical engine to get it into space. It's hard to make the numbers work -- nuclear-thermal may be more efficient than that chemical engine, but it's dead mass while you're boosting it out of the Earth's gravity well. If instead you can use the same chemical engine to get to Mars that you used to boost yourself out of Earth, you don't have that dead mass. If you want to get to Mars fast, just boost some extra fuel on an extra ship.
Nuclear-thermal looks nice on paper; it's probably the highest efficiency high thrust engine that's achievable with today's materials. But it's hard to find a good use case for it. A manned Earth->Mars run used to be it; but SpaceX's plan for in-space-refueling shortens the trip time just as much, if not more.