Regarding Saruman’s motive for turning, what I’ve carried away from the various sources is that Saruman/Curumo had deep seated self-image issues. He saw the mission to Middle Earth as his chance to prove himself; hence his jealousy of Gandalf and his increasing disparagement of both the elves and the heirs of Numenor whom he saw as frustratingly useless to his goal of being the big hero who defeated Sauron. He was determined to make it all about him, whereas Gandalf simply did his best never even hoping for a guarantee of success.
I think this is also due to the author (a veteran of the Great War) holding a profound dislike for “technology”.
This quote from “The Hobbit” is telling:
It is not unlikely that they [goblins/orcs] invented some of the machines that have since troubled the world, especially the ingenious devices for killing large numbers of people at once, for wheels and engines and explosions always delighted them,
Yes.
Tokien was not enamored of technological advancement. In his youth, he watched the industrialization of Birmingham and environs eat up the forests and small farmsteads he loved. As a young and not especially gung-ho communications officer in WWI (the first truly “industrial” war), he watched the fruits of technological progress kill dozens of his friends and thousands of young soldiers on both sides.
That’s the reason that Saruman is painted, not just as a formidable wizard, but a talented and ruthless industrialist.
Saruman’s final spiteful play in the Shire is considered to have been a retelling of the fate of the lovely wilderness he grew up with, and the Scouring wish-fulfillment about driving out the human orcs that changed the Midlands into the Black Country.
Yeah, I was tempted to respond to the OP just by saying “Because that’s the way it happened!”
If the Red Book of Westmarch says it happened who was he to change it?