Bomb fission has an extermely hard (fast) neutron spectrum, so relatively few long lived isotopes are formed. Thus the steep dropoff. in another ca 350 years, almost all of the decay chains will have run to completion, and the only readily detectable isotope will be technetium.
The fact that we deploy soft spectrum fission in power generation is kind of daft - The very long term toxicity of nuclear waste is wholly a product of the fact that most reactors use thermalized neutrons.