Why? I have 3D Viewer and Java8
It depends on your graphics card and platform. Some of them are broken now; see fiji/3D_Viewer#17 and fiji/3D_Viewer#18 for details. And some platforms have really obnoxious bugs, like the 3D window going dead—or even crashing ImageJ—if you try to resize it.
The best case scenario for usability with Java 8 right now is if you can install Java 3D 1.5 into your Java 8 JRE; this works on some platforms but not others. But then you have to have the Java-8 update site turned off since we already migrated all the Fiji components to use Java 3D 1.6, which has a different package prefix.
I am very sorry I do not have time to fix these problems, after inflicting this situation on everyone by upgrading Java 3D to the experimental 1.6 version. The alternative was no working Java 3D at all on some platforms though, as well as dubious legality in the first place of redistributing the old dead Java 3D 1.5…