Hiroshima and Nagasaki were air bursts. Explosive, lots of destruction, big plumes, and a fair bit of fallout contained in them.. The fires it set were not inherently radioactive, but they did spread the limited amount of fallout produced by the singular explosion. Chernobyl was a ground level reactor meltdown. Smaller area, smaller plume, not as explosive, but it kept burning for DAYS, and ALL of the plume was made of radioactive material. It just kept spewing out radioactives until the cloud spread across national borders and everyone realized something was up.
Also, the Fat Man and Little Boy bombs were extremely crude and each only actually fissioned a tiny portion of their cores, and that when a bomb goes off, it has only a couple of milliseconds to undergo fission before it destroys itself.
See the report I linked to above.