Why is one year shorter on planets close to the sun and longer on outer planets like Uranus?

It comes down to 3 main factors - associated with centrifugal/centripetal force plus gravity:

Firstly, is simply the much smaller path required for a planet closer to the sun to travel in its orbit of the sun. Naturally, this will mean that (assuming the speeds of travel through space to be the same),the lesser distance to cover will result in a shorter year.

Second factor: In order to prevent gravity from pulling a body closer to the sun in a collision course, the outward force/momentum propelling it away must be more marked.

Hence the revolving object must travel faster through the air in a more outward direction as if 'away' from the object it is orbiting.

Third factor: The size of the planet will also affect the overall 'drawing power' between the 2 heavenly bodies. If Both objects were large, the overall gravity would cause that they close in on one another.

If the planets close in were as large as Uranus, there would be certain collision unless the velocity through space of the large planet was many times that of a smaller planet at such distance from the sun. If Uranus replaced Mercury, for instance, the planet would have to travel at some speed many times faster than that of the speeds of those smaller planets in closer to the sun, and a large planet that close might affect the stability of the sun's traverse through space.