Why do the Jovian planets rotate rapidly?

That's actually the "natural" rotation speed for planets. We small puppies (Earth, Mars, etc.) are so light, it's easy to slow us down.

Think of a merry-go-round and a roulette wheel, each spinning at the same angular speed (same number of revolutions per minute.) Now start hitting them each with random debris, minor gravitational perturbations, etc. Which one is going to be affected more?

Silas

What slowed the Sun down?

The Sun got slowed down because its surface is convective, and a convecting ionized gas tends to produce magnetic fields. These fields get dragged out by departing material, which gets lower and lower angular velocity on account of its getting some linear velocity away from the Sun. The magnetic field functions as a sort of connection, and the Sun tries to pull this material in the direction of its rotation. As a result, the Sun's rotation gets dragged, and it slows down.

Other stars show evidence of this effect; there is a break around F5 in the Main Sequence; more massive stars have radiative outer layers, and thus remain rapid rotators, while less massive stars, like the Sun, have convective outer layers, and thus become slow rotators. Additional evidence comes in the form of starspots and similar analogs of solar activity; less massive stars have such activity, while more massive ones do not.