I've already asked that question.

Fabrizio may have been promised relocation to the USA by Barzini as return for killing Michael in Sicily. In fact, because it was Appolonia at the wheel, Fabrizio failed in his attempt. So why bring him over then?

Since according to the Saga it is nearly 10 more years before Fabrizio does end up on American soil...one can assume that he continued to do other favors for the Barzini people, ones that were more successful, before finally reaping his reward.

Apple