It's a U.S. federal crime to conspire to kill a person in a foreign country. I guess they could have been extradited but since they were facing life (and in the federal system there's no parole), the Philippines didn't insist on trying the hitmen themselves.
Indictment: https://www.justice.gov/usao-sdny/file/632661/download
Relevant statute: https://www.law.cornell.edu/uscode/text/18/956