Annoyingly, a few authors decided to jump the gun and publish books around PHP 6. I think this was the main deciding point. On the one hand, it might be confusing for a newcomer who thinks that they're getting a book on the latest and greatest; on the other, perhaps they should check publication dates.

Either way, it wasn't a particularly important decision - any number signifying a major version bump, greater than 5, would have been fine.