Kevin Costner also touched on The Bodyguard, noting the studio originally wasn’t keen to hire Whitney Houston because they wanted Costner to “look at established actresses.”
In the same vein, Princess Diana was not established as an actress either, although she had been interviewed.
Whitney Houston was not an established actress at the time and she did well with the role.
It sounds like Princess Diana may have been a good fit if she had committed to the role, given the success of the original movie.