Any info/details on scene management of jmonkeyengine please? or any sites to read on? …wish to know more about it… ^^
coz scene graph is a very commonly used technique for 3d engines. it simplifies object position and movement, and also enhances the performance of the engine since the bounding volumes of objects are also in the hierarchical scene graph which makes culling check easy.
portal/zone method is also commonly used but its good for indoor scenes only and i personally believe that a good scene graph can be even better than a portal/zone system.
my guess is: because it works.
(ok, i don't know)
Your best option would probably the book by David Eberly. jME is based on the concepts shown in there.