Why did the spanish want to convert american peopls to christianity?

Catholicism is Christianity, separating it is like separating food from pizza. 

The Spanish wanted money. The Church also came along and (to a degree) protected the natives (at least it offered more protection than the Protestants did to the North American Natives). There were also many missionaries that spread the faith. They succeeded. The Spanish and natives interbred and that is the decent of a majority of South Americans (unlike Protestant settled lands where almost every native was slaughtered).