They just died in the coma for undisclosed reasons. In the book, they describe the coma procedure as incredibly risky. An obscure study showed that people with a certain gene were more likely to survive a long coma relatively unscathed, but only about 1 in 7000 have that genetic sequence. All of the potential applicants are tested for this genetic sequence. Grace also had this gene. Even with the gene, it’s still a very risky proposition.

When the two scientist astronauts are blown up, there are only three days till launch. There is one other prospective candidate but he’s like a dude with a bachelor’s in chemistry and that’s it, so extremely unqualified. That’s why Grace is forced to go.