Actually, I believe that you'll find Tolkein does explain the whereabouts of Orcrist, but he waits until partway through The Fellowship of the Ring to do so.
A quick search on Orcrist at Google (www.google.com) reveals the following information from the Encyclopedia of Arda page (www.glyphweb.com/arda/default.htm?http://www.glyphweb.com/arda/o/orcrist.html):
Exactly what happened to Orcrist at Thorin's funeral isn't completely clear. The Hobbit tells us that the sword was placed by Thranduil 'upon his tomb', and that it 'gleamed ever in the dark if foes approached' (The Hobbit 18, The Return Journey). These quotes seem to suggest that Orcrist wasn't actually buried with Thorin, but instead just laid on his tomb.
In The Lord of the Rings, there's a suggestion that this may have changed. We are told there that Orcrist '...lay now upon the breast of Thorin...' (The Fellowship of the Ring II 3, The Ring Goes South). This seems to suggest that the sword was actually placed within Thorin's tomb, and not just placed on it, though this text is admittedly ambiguous.
This seems to agree with my rememberance that Orcrist is entombed with Thorin