In brief: NERVA was an extremely expensive, although promising, technology that was not quite ready for flight when NASA's budget was cut from 1969 onward.
Even as early as 1965, budget line items for NERVA (and related projects like the S-N upper stage) were understood to be proxies for a later Mars project, which was controversial at best, due to the extreme expense at a time of substantial US budget difficulties. NERVA was generally planned to be an essential part of the manned Mars missions circa 1978, but after LBJ left office the political support for such a massive and expensive project evaporated.
(To The End Of The Solar System: The Story Of The Nuclear Rocket has lots of good reading)
Nuclear propulsion has remained a source of interest in rocketry, but a NERVA-type high thrust engine is of limited value with current spaceflight objectives. They are extraordinarily heavy for an upper stage engine, as well as posing special safety hazards. The vast majority of current spaceflight has no practical need for a heavy high thrust maneuvering system when a lighter high efficiency system can get a probe around the solar system for far less money.
ETA: they also blew one up in 1965 and it got radioactive material all over the test site, which didn't exactly help the popularity of the program.
There is indeed interest in developing nuclear powered thermal rockets, but the issue is complex.
Nuclear engines have a few problems - they are heavy, they require shielding (which adds to the weight), they require cooling, and they are dangerous in case of catastrophic failure.
Considering their weight, you would think the best use for them would be on the lower stage of rockets. After all, upper stages are dead weight until the rocket is already suborbital, and why would you want to carry a heavy engine as dead weight when you could use that mass for extra propellant or more payload instead?
But there are problems on using them on lower stages:
Lower stages are typically jettisoned when they run out of propellant. You don't want to jettison several tons of highly radioactive material.
They aren't very powerful. Lower stages on rockets require extreme power for very brief periods of time. Nuclear engines aren't designed for that. For example, the first stage on the Saturn V had five F-1 engines that delivered about 12 GW each. That's a total of 60 GW for stage one. The biggest land-based nuclear power plant on Earth (Palo Verde) only delivers about 3.5 GW. Granted, Saturn V was huge, but you can see the mismatch when a rocket engine requires almost 20 times the power of the biggest nuclear reactor on Earth. Without the benefit of millions of tons of water from a nearby river for cooling, or thousands of tons of concrete for shielding.
So given these problems, they are unlikely to be used for lower stages unless we build some kind of fully reusable giant ship, which is a scifi scenario at present.
Returning to their use in upper stages, as I mentioned, they are inefficient compared to chemical rockets below some minimum delta-V requirement. What that means is that since they will be heavier than chemical rockets, in order to make up for that extra weight and still come out ahead, the mission must require a lot of work from that engine.
Most missions don't require that much work from the upper stages. Orbital station keeping or boosting the orbit only use a small amount of delta-V. Interplanetary missions require more, but interplanetary missions typically have tiny payloads, and they are quite rare.
Ideally, nuclear powered thermal rockets would be useful for high delta-V missions, such as sending large payloads over long distances in the solar system. In fact, that was the original intended use for NERVA -- to power a shuttle that transported people or payload from low Earth orbit to Mars. And it fell through when it became clear that nobody was going to Mars anytime soon.
So that is the situation at present. The benefits of nuclear powered thermal rockets are most pronounced when you have some fairly regular traffic involving relatively large payloads over interplanetary distances. When such missions are seriously under planning or become routine, we will probably have nuclear powered thermal rockets.