I figure it's probably just some kind of miscommunication somewhere as to what's supposed to be promotionally free, and when.
Sometime places get get skipped for a title or two, but then go free on the repeat, or a book shows up later, like with that
KJV Apocrypha freebie that missed iTunes on the 1st round, showed up there for the 2nd, and then made it to BookShout on the 3rd.
Given that Baker does frequent 1-day-only freebies, something's bound to go wrong somewhere at some point, and it's probably just some kind of statistical clustering that it's more noticeable with them this week (and we did start to get Kobo & Google Play back after a few skips, so ChristianBook will probably start reappearing shortly).