Why there is a systematic decrease in the synodic periods of the planets from Mars outward?

The systematic decrease in the synodic periods of the planets from Mars outward is primarily due to their increasing distances from the Sun and their orbital speeds. As planets move farther from the Sun, they have larger orbits and slower orbital velocities, which results in shorter synodic periods relative to Earth. Additionally, the gravitational influences and dynamics of their orbits contribute to this trend, affecting how quickly they appear to move across the sky from our vantage point on Earth. Consequently, outer planets exhibit shorter synodic periods compared to those closer to the Sun.