Because she was 19 when they married. Also, because it was arranged. Liz likely knew she could bend little naïve Diana to her will and hopefully keep Chuckie away from married horseface long enough to have some heirs. Diana was crying the day of the wedding because she found out he was on the phone with his mistress shortly before the ceremony. Too bad Diana didn't have the fortitude to tell them all to take a hike. She might be alive today if only she had.
I’d heard that she had found out about the affair right before the wedding. Being so young & all the pressure on her about the wedding, I’m not blaming her for going through with it.
I always had sympathy for her. She grew up in a wealthy but incredibly dysfunctional family, where she received very little love. I guess that’s why she identified with the less fortunate people.
I wish her life could have had a happier ending.