An international background can be much more expensive and take much longer than a standard background check. It is much more complicated and there are more forms and hoops to dive through.
Some of the challenges and complexities stem simply from differences in culture and geology. That can include the way your candidate’s name is spelled, differences in terminology for criminal records, time zone variations, etc.
The international background check is not governed by the same laws as those performed in the US. Here, candidates are protected by the FCRA, but internationally you have other types of laws to contend with: human rights and labor laws, privacy laws, data protection laws, etc. that vary by country.
In some instances, researchers have to go directly to police stations, not just courthouse jurisdictions, to obtain case information.
All of these elements cost money and take time. Hiring anyone from another country is big expense, but skipping out on the international portion of the search because of the cost and expense of time is dangerous. You are expected to vouch for the people you hire, so it is important you use all the tools available to you to make the best decision possible.