If you search, 5ivedom has provided a pretty good explanation of why some apps aren't compatible and it seems to have to do with how the developer has set up the app. But his explanation is much more clear.
Blackbear, when developers make apps for Android they either specify
1) What class of apps can run it (high definition tablets, phones with this screen density,etc.).
2) They specify a specific list of devices that run the app.
Since Nook was not open to Google Play store, it's quite likely that
a) Some Apps simply didn't include the criteria for Nook HD and HD+. Keep in mind these have very high screen resolutions. So they aren't the same as Nexus 7 etc.
b) Some Apps include Nook as 'not supported' or don't include Nook in the 'supported devices list'.
In either case it would show that 'App is not supported.' The message you saw: "This app is incompatible with your BN LLC BNTV600"
Best to contact the developer. Over time as developers see sales from Nook devices and find out Nook devices support Google Play, more will support Nook.
Keep in mind that developers found out same time that you did. So it might be weeks to months before they can get Nook, test and publish.
In some cases they could do a quick fix. In some cases, not.
the app in question is a planetarium app which can use gps, compass and accelerometer to update its point of view.
the dev may have sensor requirements that it doesn't meet.
for what it's worth, I think this would. perform badly on the hd+ -- on much faster hardware with a lower res screen [1280 x 800 on galaxy note 8] I tried the app out and returned it for a refund. I like astronomy apps, I have both vortex and star chart in my library, but pretty as this one is, it's very slow.