The Netherlands midfielder said his decision was influenced by both his role at City and the financial opportunities available in Saudi Arabia.
Reijnders became concerned during pre-season that he would again be used in a more advanced midfield position rather than the dynamic role he preferred, contributing between defence and attack.
"I started thinking about whether I wanted another season like that," Reijnders said. "The honest answer was no. I wanted to look elsewhere."
Reijnders initially had doubts about moving to Saudi Arabia, believing such a transfer could be viewed as a step away from elite European football.
Those concerns were eased by Al Qadsiah manager Brendan Rodgers, who told him he could enjoy several seasons in Saudi Arabia before potentially returning to Europe.
"That is my goal too," Reijnders said, noting that other established players had already taken similar routes.
The 28-year-old also defended the standard of the Saudi Pro League, arguing that people should watch its football before judging it.
Reijnders was also candid about the financial motivation behind the move. Reports have put the value of his Saudi contract at around 20 million euros per year.
"You can always play the moralist, but I think everyone would say yes to that," he said.
Reijnders explained that the deal gives him the opportunity to secure his family's financial future after his playing career ends.
"I don't want to have to work anymore after my career," he said. "Now I can also set up future generations of my family well."
While his move has taken him away from Manchester City after only one season, Reijnders has not ruled out eventually returning to European football."