It has to do with the amount of radioactive material and half-life.

From wikipedia:

The Fukushima plants have tons of nuclear fuel, thousands of Fuel Assemblies, more than 6,000 fuel rods in Spent fuel pools. ---

Fukushima: 1500-2000 tons of spent fuel rods

Little Boy: 0.07 tons (140 lbs)

Fat Man: .0068 tons (13.6 lbs)

http://news.sciencemag.org/scienceinsider/2011/03/how-much-f...

There is more radioactive material to release from a reactor than a bomb - see http://en.wikipedia.org/wiki/Comparison_of_Chernobyl_and_oth....

Whilst a bomb will cause an area to become radioactive itself (through neutron bombardment) that is confined to the local area and I think tends to have short half-lives.

I may be completely ignorant as I'm not an authority on the subject, but I think it's the difference between the radioactive materials being exploded/vaporized vs shot into the atmosphere (without being vaporized/broken down) and distributed in the general area.

With the the A-bombs, the radiation that people suffered wasn't from the radioactive materials in the weapon itself, but the radiation from such a huge explosion.

With Chernobyl, the radioactive materials were sent into the air and settled around the vicinity, making many parts of it still very radioactive to this day.

Pretty much, but the largest factor was simply the sheer difference in scale between the A-bombs and the reactors at Chernobyl. With the A-bombs there was simply simply less fission fragments and other radioactive products to worry about due to there being far less nuclear fuel and surrounding material that could be irradiated and itself made radioactive.

Meltdowns have the potential to release far more radioactive material than an atomic bomb. The radioactive material tends to consist of isotopes with high nuclear decay rates. And the material tends to stick around longer, slowly poisoning the environment, as opposed to a bomb blast which quickly scatters the material into the atmosphere.

The fallout products of an atomic bomb generally have short half lives. Wikipedia has some interesting graphs comparing the radiation dose of a fission bomb and Chernobyl:

http://en.wikipedia.org/wiki/Nuclear_fallout#Half-life