JUICE has a launch mass of 5,963 kg while Juno had 3,625 kg and Voyager only 721.9 kg. I guess it may be a limit of the launch vehicle and the mass the spacecrafts have.
So NASA has the choice of 1) launching a lighter/smaller probe with less scientific capabilities and endurance, 2) spend $$$ and time developing a much more powerful launch vehicle, or 3) have a few years of patience waiting for JUICE to get to Jupiter on a slower trajectory.
Jupiter has already been visited by multiple probes so sending a small and not very capable probe would not gain us much scientifically. (But this option was taken with the New Horizons probe to Pluto some years ago.) (2) Wouldn't actually be faster and much much much more expensive (see how long the SLS rocket is taking to get done. But mr. Musk is taking this route too with SpaceX's Starship, so this will be an option in the future if all goes well). So that leaves option 3, which NASA has taken. They could probably have opted for an existing more powerful and expensive launcher to shave some years off the transit, but they preferred to wait a bit longer and not spend that money.
[1]: http://www.astronautix.com/a/ariane5eca.html [2]: http://www.astronautix.com/t/titaniiie.html
> So that leaves option 3, which NASA has taken.
As far as I'm aware, JUICE is an ESA probe. NASA is providing one of the instruments, but I believe that's it.