The short answer for why Juno is solar powered rather than nuclear power is because we can.
At the time we did the proposal for Juno, NASA had, generally peaking, a policy that it's okay to use nuclear power to do things in space where you need to use nuclear power
but if you can do something simpler like solar panels, you should.
We decided it was probably less risky to advance the technology of solar cells to work at Jupiter than it was to invent a new nuclear power source.
We haven't taken solar panels that far before and run an entire spacecraft that far from the sun off of solar power.
We had to work at a colder temperature, we had to work with less light, and we had to be able to work inside a radiation environment
so as things got damaged you had to either protect them or make them more efficient.
The biggest design challenge of the solar arrays was probably just their size.
Each solar array is 28 feet long, and so we have three of them, and so when the solar arrays are fully deployed, the JUNO spacecraft is almost 60 feet in diameter across the solar arrays.
Getting those big solar panels to work the way we expect them to work, and produce the amount of power we need, that's been a bit of a challenge.
The solar arrays are pretty interesting. When we deploy them, they'll be generating about 12 kilowatts of power.
As we get further and further from the sun that amount of energy will drop off until we finally get to Jupiter where we're only generating four hundred watts.
It's not even enough to run a hairdryer.
Generally, with space exploration, you're pushing the envelope of technology.
Solar panels have improved a bit, the instruments have improved a bit and can run on less power, and I think it's turned out to be a very good decision for Juno.