Could they also be holding off because Samsung hasn't settled on a perm design yet?
Not sure when they'll start selling for most of the post, but this part - the design has been final for months at this point. They got their FCC certification last week, they've been going through carrier certification, final software testing, etc.
Just as a glimpse into how the timeline works, the original design usually begins about 16-20 months prior to the device launch. During that time they can start with preliminary conversations about what components are going to be available, where they're going to produce, get the preliminary design going, etc. A lot of those component decisions are dictating the design of the device.
Semi-final specs and the secondary design phase are typically determined 8 to 12 months prior to the release. At that point we usually have somewhat working prototypes running very basic software to simulate any new features, functionality, etc.
By the time we get to 6 months before launch, final specs are typically complete and parts have begun being ordered, assembly line time is scheduled/ordered and most of the software for the device is complete.
3 months prior to launch, it is almost impossible to change any hardware decisions unless there is a showstopper that is going to justify delaying the launch.
1 to 2 months prior to launch, assembly has begun on the first batches of units and units are being stocked up. The closer they wait to the 30 days or less period, the more up to date the software is out of the box.
As we're now in that last period, it is safe to say that the final design is set because the first units have already been fabricated. All that said, Samsung has a bit of an advantage in that they create a lot of their own parts - so they may be able to slim down the timeline a bit. Also, this timeline is based on how things worked a couple years ago, it's possible that the whole process has been slimmed down even more, but this is generally why we see new features/trends in devices emerge over a 1.5 to 2 year period rather than everyone doing the same thing all at once.
TL/DR, the Note 8 was started before the Note 7 ever was announced and the design was almost definitely "final" before the S8 launch.