All I know about him is that he's a famous soccer player who surely must make an insane amount every year from endorsements. My wild guess - $50 million a year? Maybe he didn't want to seem to be "political" to lose a dollar of that sweet, sweet cash.
Messi is the most popular human on planet Earth. He is unironically a bigger deal than the President of the United States. Probably just didn't care to go.
Messi is a big deal, he probably didn't want to be associated with the genocide financier and the cynic in me says it's because it would hurt his brand but he is probably wrong