The Voyager missions were able to use a rare and convenient alignment of the planets that would slingshot them into deep space. Juno is orbiting Jupiter itself,

The Voyager missions were able to use a rare and convenient alignment of the planets that would slingshot them into deep space. Juno is orbiting Jupiter itself, which is not a hard thing to do in our solar system. JUICE is attempting flybys of several of Jupiter moons, to end up orbiting around Ganymede. This is a much more delicate orbital insertion and has much stricter requirements for the gravity assists on its way there.