They pay Ronaldo well because he's not only a good player, but also a global promoter of the league.
They prefer this league because it offers good pay for established star players. Perhaps in 3-5 years, younger players will join the league as well.
The Arabs are very wealthy due to their oil resources, and if they continue like this, they will make this league one of the leading leagues in Europe.
I think Al Hilal also did quite well in the FIFA Club World Cup.
I think there is a grand portion of credit which directly goes to Cristiano Ronaldo to make this league as popular as it is today. Because the following of the Ronaldo is beyond the borders. People love him overall in the world. And i think the main purpose of hiring Ronaldo with heavy wages is only to attract the world towards this league. Because since Cristiano Ronaldo is the part of Al-Nassr there are no significant titles or goals scored by Ronaldo. But still the management wants to keep him the part of their club is only the indication that he is in the league as a pull factor for the world. And as per rumors if Al-Ahli gets success to sign messi then i think this league will be unmatchable. Because both are great players with huge fan following.