It's because of the potential. Had Chernobyl gone extra wrong (eg had the people in charge there not responded correctly), millions could be hurt, and not just in the Soviet Union. It also dominated the news for months in Europe, with the memory of having to throw away certain homegrown vegetables due to radiation being engraved into the collective consciousness.

Additionally the Bhopal catastrophe happened to Indians and people in the West generally care little for them, due to racism and distance.