this is an engineering problem, and incompetent people should have nothing to say about it. One of the primary ways that we can identify incompetence is when someone say if we would have done this then the problem would have been solved. Engineering problems are complex, especially in space travel, and there is no way to know that, for example, a nuclear power source designed for a soft landing would have survived a hard landing. That is, in fact, the engineering problem on which the mistake was made after all.
To answer this specific engineering problem, plutonium is simply too dangerous and costly to use in space. The reason is that plutonium is actually very safe to humans except when breathed in as small particles, such as what might be generated when a launch vehicles catastrophically explodes on launch. In this case, the small particles will tend to be inhaled by animals, pass through the lungs, and pretty permanently become part of the body. The plutonium will then go though the 24,000 half life, which means over the lifespan of the contaminated human almost no Pu will decay. It will radiate and cause health issue for a lifetime.
Again, this is an engineering problem with very smart people working it. All engineering problem result in an engineering solution, and an engineering solution is always a compromise between competing factors, some technical, some emotional.
In hind sight it is always easy to poo poo an engineering solution. People who do nothing but push paper, like the readers or forbes, are the most likely candidate is simply say 'why did we do this'. They can ask that question because they have never created a practical device in their lives, therefore never have been part of the engineering process and therefore have never understood that the result is always a less than perfect but usually quite acceptable solution.
While the nuclear power proponents want us to believe that nuclear power is the solution to everything, history tells us otherwise. Even though nuclear power is very mature technology, there is little private funding for it. In the US Nuclear power plants are not being build because bankers know there is no profit in it, and government should no more subsidize a nuclear power plant than a coal fired plant. Both are mature enough to stand on their own.
Nuclear power cannot stand on it's own because it cannot generate enough profit. For instance, BP generates enough profits so that when the Deep Horizon rig failed it could cover the 13 billion dollar clean up. Fukushima is going to cost 10 times that much to clean up. Who is going to pay for that. They taxpayer. The US taxpayer for contamination that reaches US land and water. It is true that the readers of Forbes loves to make profits at taxpayer expense, but I don't think that it is a good idea. It is only free if you are not the one impacted.