Why did people lose interest in space?

I haven't bothered to check the numbers, but I think it might be very difficult to do that, with the rockets we have today.

It helps a lot that Uranus and Neptune are so massive and have moons, but probably not enough.
A fly-by is far more doable - which is why we see those -, than going so far out in the Sun's gravity well, and then injecting into an orbit around a planet.

The same is true the other way, inward, deep into the Sun's gravity well. But they managed to sort of orbit Mercury a few years ago. It wasn't stable and eventually crashed. But they had an advantage in that they could use substantial planets like Earth, Venus and Mercury for a 7 year long series of gravity assists.