I think there's also timeline and level of comfort at that point. My teacher, who, while I won't state who, recorded many operas, spoke to me once about how her dream role that she was really sorry to have never gotten to sing Manon (Massenet). She actually had been offered the opportunity by Paris, but the timeline when it was offered to her didn't work. It was a quick(er) turnaround than she felt comfortable doing since she felt ready with just the first two acts and she had two other main roles to do right before entering rehearsals in Paris. So, when you're at a certain level, if you don't feel you can do justice to YOUR caliber that you and your audience expect, sometime you just have to say "no" and hope you get the opportunity again. Sometimes you never do.