He needed a visa to enter the USA. He couldn’t just buy a last minute ticket and jet right over. He wouldn’t be able to get a visa in couple of days.
He was exposed on Sep 15th and flew on the 19th
http://www.avianflutalk.com/he-knew-he-had-been-exposed_topic32242.html
unless he was exposed earlier - like the victims who died
Pure speculation - he may have been symptomatic when he flew, bribing his way past the thermometer scan at the aiport
He had family here. I can’t claim to know the intricacies of tourist visas for Liberians coming to the US, but it’s not inconceivable that he already had one.