As far as I know, this is up to the individual developers of those apps, who made a judgment call as to whether they can support the oldest device in the Gear VR lineup. Personally as a developer, I love the Note 4 as that's "where it all started", but I find it can be very sensitive with regard to texture memory usage compared to the newer devices.
Titans of Space PLUS for Quest is now available on DrashVR.com
The reason some apps may work on only some phones is purely technical. Each phone may have different components, and a game built to run (let's say) on an S7 may not work well on an S6. It is up to developers which devices they wish to support. Some may spend time to optimize for older phones but some may not. Hope that makes sense,