The exclusion zone isn't unnecessary. It's gone. All exclusion zones have been remediated and reopened: https://www.cnn.com/2022/08/30/asia/futaba-fukushima-nuclear...

Nobody bothered to clean up Chernobyl because the whole town's existence was to support the plant and plant workers. With no plant, the city had no value and thus no incentive to spend money on remediation. By comparison, Japan showed us what occurs when places with value are contaminated: effort is made to clean them up. Half of the exclusion zone was reopened within 5 years, and the most intensely irradiated locations were cleaned up in just over a decade.

This doesn’t fully track, the Chernobyl popwerplant was in operation for many years after the catastrophe. I think the sinister truth is that the soviet union just cared a lot less than the Japanese government, not to mention the slightly bigger scale of damage.

More specifically: Pripyat and the original exclusion zone evacuated. Slavutych [1] was subsequently constructed to support the nuclear power plant after the disaster. Because again, Pripyat had no harbors, and no real economic value other than supporting the power plant. So it was simpler to just construct a new city.

1. https://en.wikipedia.org/wiki/Slavutych