Other than obvious physical traits:

Power of observation. Accurately seeing the game(s) and your opponents lets you anticipate, which is basically partially seeing the future and it of course is incredibly powerful.

Analytical mind: observing is the first part, then analysis to understand how or why you can take different approaches to win, attack, defend, etc. is part two. There are almost always another level of analysis to be done.

Curiosity: most people plateau physically long before they’re mental capacity in a sport is exhausted, but even at the pro level you still see many athletes that once they’ve made their paycheck or won x, they coast or stop developing. True multi-sport athletes are curious and diversely talented and this drive to understand helps them not just be satisfied with a single focus or sport.

I don’t think anyone has mentioned: being very competitive.

Finding sports that have significant overlap.

Someone who is good at the 100m will likely also be good at the 4x100m relay.

Michael Jackson played multiple pro sports?! The king of pop-ping off on the sports-ing field.

Im not even sure what you mean. So you mean what makes an athlete able to compete at the highest levels in multiple sports?

Not really possible. Play sport at top pro level requires such a specialization than will not work out for any significantly different sport.

Take triathletes for example. The best ones still suck at any of the three sports individually…

A top swimmer will have a totally different body from a tip cyclist or a top runner.