Perhaps trying to deflect criticism of the Vision taking money away from science missions? I think it might have been a factor in the decision anyway. Juno IS a very cool mission though:

For the first time, instruments will peer down through Jupiter's opaque cloud tops to not only confirm the existence of a rocky center, but also to measure the planet's internal convection at exceedingly high atmospheric pressures deep below the visible surface. Juno also aims to detect the amount of water, ammonia, and methane in Jupiter.

Scientists also plan to learn more about the temperatures and wind velocities at different levels in the Jovian atmosphere, and to further characterize the planet's huge magnetosphere that traps harsh radiation that could pose dangers to future spacecraft.

Ben Bova's novel "Jupiter" theorizes the solar system's largest ocean existing as a 100 mile thick layer of water completely surrounding Jupiter several hundred miles below the cloud levels. Would be fascinating if true.