why didn't they just ask Lauren/Sylvia to be the wife?

No they couldn't use someone that was genuinely in love with him because it was too risky. They felt they had to keep Truman under control to keep the show going. And she DID try to tell him about the show, then her "father" whisked her away. They needed a wife that didnt love him

Also if you notice the current day Sylvia was some sort of "FREE TRUMAN" activist hence the posters all over her apartment and the phone call with Cristoph. She felt sorry for him. She wasnt as cold as his "wife".

I read this in a chola accent.

you have to also imagine they are trying to attract viewers and "Hannah Gill"/Meryl is a bigger name than Sylvia blahblahblah

Imagine if someone like Jennifer Aniston was cast as the wife the audience would be a lot more interested in the bigger name than if Truman fell in love with some up and coming actress that nobody had heard of yet. With the big name comes headlines, publicity, more viewers which leads to more advertisement revenue etc. etc.

Its where Christof is completely in the wrong where he believes he has created a 'real world' for Truman. Where in reality he was just as fake as all the other Hollywood types he disliked so much (his motive for starting the show in the first place).