Why nuclear criticality accidents weren't explosive like an a-bomb?

Answer 1

The fissile material involved in all of the recorded incidents did not have sufficient density to detonate. In most cases, the heat generated by the reaction caused the material to expand enough that the reaction became subcritical again. Fission bombs use conventional explosives to quickly compress the plutonium core to a very high density so that the entire mass becomes prompt-critical before the core can expand.