Jupiter is a relatively fast moving planet. Of the planets outside the earth's orbit, only Mars is faster. But Mars is so clearly red (and usually dimmer than Jupiter), so its role seemed more warlike. The two inner planets are restricted to orbits closer to the sun, and of these only Venus can attain any remarkable brightness. Jupiter in opposition is fairly bright, and it ranges freely throughout the heavens with a good pace. As a matter of deduction, it must have seemed to be the king of the planets.

hi! you might be interested in this earlier thread

• Did ancient Greeks know that Olympos was the highest mountain in Greece when they placed their gods there? Did they know Jupiter is the largest planet when they named it after the king of gods, Zeus?

these threads about the names of planets may be of incidental interest (there are scattered mentions of Jupiter, but none really address the fact that it's the largest planet)

• Why were the planets named after the gods the ancients named them after? More specifically, why was Saturn given his own planet instead of a more... prominent god?

• What was the relationship between Ancient Greek religion/gods and the planets named for them? Did they think Venus, Mars, etc were the actual gods?

• When did scientists recognize the planets in our solar system as (relatively) nearby planets, and not as other stars. How did they know that these celestial bodies were planets and not stars?

• Why did planets get named after Roman Gods and not Egyptian, Greek or Norse?

• When ancient people named the planets after their gods, did they really believe that what they saw in the sky was that god, or did they know it was just an object named after them?