Once its initial incarnation and the Mars program was cancelled for cost reasons there was no longer a need for anything like it. As the upper stage of a Saturn 5 it would create a much more powerful rocket than what we used to go to the moon; but with Apollo being wound down and no successor in store there wasn't anything it would be beneficial for to resurrect it.
I'm not sure where your 3:1 thrust to weight ratio came from; while I've seen multiple claimed performance numbers for what Nasa achieved before the program was cancelled; but none of them had a rocket stage with a fueled thrust to weight ratio greater than one, leaving them only usable as upper stages. The highest I found was in Encyclopedia Astronautica (bottom of the page) which listed a fueled mass of ~178 tons and an 867kN thrust for a thrust to weight ratio of 0.5. Wikipedia's description of what otherwise appears to be the same system lists a much lower thrust of 330kN for a 0.2 ratio. This leaves it unusable for anything but an upper stage.
Even if modern designs could get a thrust to weight ratio slightly above 1. the political impossibility of dropping reactors from spent stages would mean that to use it as first stage you'd need performance high enough to also carry hardware to make a safe landing afterward or to serve as an SSTO. These days I suspect nuclear distrust would make even using it as an upper stage impossible under any but extraordinary circumstances.
I know a man who worked on it. All my knowledge of the nuclear program comes from him.
The design objective was a single engine to be reused (fancy that--they're talking about reusable rockets in 1970) for a large number of TLI burns. Specifically, they ignited the test engine on the ground 10 times.
He says it was cancelled due to a lack of a mission when Apollo shut down and there wasn't any moon program anymore. It was intended for Apollo's successor, which never happened.