If the moon is sighted anywhere in the world, it is considered acceptable for the rest of the world to welcome a new month based on this sighting. That is according to many Hanafi scholars. There are also some scholars that hold the opinion that the moon has to be sighted locally. The other reason people do not follow Saudi is that they (including myself) believe that Saudis are not trustworthy in their sighting claims. Or they have some eyes that are able to have selective kashf and see tomorrow's moon today!