Re: Destiny trilogy - pretty heavy spoilers
1. Because they're idiots
2. They haven't got permission yet.
Or 3) S&S hasn't made the correct cover available to them.
Given S&S's proven incompetency providing material in a timely manner in other areas, I'm going with #3.
"Niceness is the greatest human flaw, except for all the others." - Brendan Moody
"...don't mistake a few fans bitching on the Internet for any kind of trend." - Keith R.A. DeCandido