I think they're not playing him because he is a traditional center, and because he doesn't fit the style of play they use when either Love or TT is at center. Also, because Lue doesn't want to do anything that would take minutes from veterans.
While I think those are the reasons, I also think they are bad reasons. Blatt shifted seamlessly from starting TT to starting Moz. Lue, on the other hand, appears to be one of those coaches who prefers a specific type of offensive style, and doesn't use players who don't fit it. He also places too high a priority on appeasing veterans with minutes.
This could have been a somewhat fun season if our two rookies had gotten significant playing time. If the regular season doesn't matter much to LBJ -- and it clearly doesn't -- then why not use it to give playing time to those who needed it the most? The entire team likely would have played with a bit more energy/effort.
Instead, we not only put a crappy product on the floor, but we took an enthusiastic, hard-playing rookie in Cedi, and turned him into a lump on the bench. Horrible coaching, horrible player development.