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.