This year is very different for the Note line. Samsung has valid business reasons NOT to give us software enhancements, while still fulfilling their basic promise to give us security and Android version upgrades. Samsung has painted themselves into a corner where their newer model is capable of fewer things, so they have to artificially cripple the older model so the new ones continue to sell.
Samsung took away too many hardware features between the Note 4 to the Note 5. In return, they gave Note 5 adopters a "premium" build and a few hardware refinements (camera, fingerprint reader, etc), but mostly many software enhancements.
They are already in a situation where many (certainly not all, but many) of their customers would rather keep their Note 4 and the features that are important to them for their own personal use cases (one or more of replaceable battery, IR blaster, SD card support, etc).
If they migrated too many of the software enhancements (many of which would work) to the Note 4, they'd further expand the number of people who would rather have a used Note 4 than a new Note 5, even ignoring the obvious price difference.
The reason Apple sends software "back" to the older hardware versions (within reason) is that they usually add or improve hardware features in each release of their phone models. In this specific case, Apple didn't have IR blaster, expandable memory, or SD card support to take away in the first place, so the iPhone (n+1) is going to be consistently more desirable than the iPhone
. So they aren't subject to this sort of "value inversion" like happened with the S5 -> S6 and Note 4 -> Note 5.
This is a temporary situation for Samsung, caused specifically by their move to "iPhoneify" their devices last year. Now that they have gone through the process of crippling the Note removing several features that made the Note line special, the Note 5 will probably get as many software enhancements as it can support backported from the Note 6. Because the Note 6 can't possibly have fewer hardware features than the Note 5 does, so the Note 6 will by definition be more desirable than the Note 5.