Spacecraft like Juno have very specific orbits to follow that are planned very far in advance in order to maximize their mission time. If, say, Juno swings too close to Io, it might get a gravity assist that ejects it from Jupiter, in which case it’s no longer very useful is it.

The closer to the object you are orbiting, the faster you need to go to stay in orbit. The faster you move, the more of an issue image blur is. Distant objects don't get much light. That means camera shutters need to stay open longer times, which also adds to blur. Yes, you would get better detail but it becomes useless because all the pixels are blurred.