Fukushima is situated on the coast line, and already leaks into the ocean, but Chernobyl is near the Pripyat river, and is a tributary to the Dnieper River, which empties into the Black Sea. The Pripyat is contaminated within the exclusion zone, but it would be bad news to disturb and agitate any contamination, and make things worse.

Given that it's already a bad situation, and that natural leeching is already taking place by doing nothing, any engineering project would have to approach the site carefully so that leeching is not accelerated.

Yucca mountain is located in the southwest desert, so that mitigates water seepage, but Yucca mountain isn't a disaster site (yet), so that technical challenge can be tackled before it arises.