If so many players have come to the Saudi Arabian league, the biggest reason for this is Cristiano Ronaldo.
it's all about money. Ronaldo himself moved to Saudi Arabia just because of the money. He would have done the same if a Chinese, Mexican, or Moroccan team would have offered him that salary.
Cristiano Ronaldo is currently at the peak of his career and as a successful football, he has won everything and have achieved unbreakable records in the world of football. A man of his age has nothing much to offer and it will be very difficult for him to compete with younger players in prominent leagues. Since he has little time left before retiring from football,I think the best option he has, is to make more money for himself and Saudi Arabian has more than enough cash to splash on him. The Honest truth is,Cristiano Ronaldo accepted to play in the Saudi league because of the money and that's is the best decision for a footballer of his age. Moreover, The Saudis are also benefiting from Cristiano Ronaldo presence in the country,his presence have attracted more investors and have also make Saudi league more popular as well.