What is the reasoning behind this? Are the Saudi referees not good enough or is it something else?
Not 100% sure. Our Hungarian refs have refereed at least 1 or 2 games in Saudi Arabia and Egypt for as long as I have been following the league. (10 years) It always tended to be the real big derby matches in part to protect local referees from threats/attempted bribery etc.
Last season Viktor spent as much time in Saudi as he did domestically - I think it was 12 games there (as referee plus others as the VAR) & 14 in our league which was mainly down to VAR training/experience/practice. They have the VAR system set up and working well there whereas in Hungary we barely have the infrastructure for more than 1 TV camera in grounds! It's not quite that bad, but it's only the last 2-3 seasons where there is now a camera in each half and maybe another 1 or 2 and they are all the same side of the pitch, certainly no views from the other side of the pitch/behind goals etc. A lot of the Eastern European leagues are similar, there's not the money in the games there yet to set up 12+ cameras to install an infrastructure capable of supporting a VAR system. Saudi Arabian league seem open to having foreign referees there on a weekly basis to learn the VAR system & gain experience & I guess in turn they get referees from leagues who are at a higher level than their own league is.