The main page seems focused on simplicity and short sentences. I think they thought that $0.99 would have looked too noisy compared to $1.
It's possible that they are also pandering to people like me who dislike prices written in that way. I am annoyed by the chore of mentally adding one to every price ending in 99 cents, and I respect sellers who write prices like that less.
p.s. surprised they didn't go for 99 cents