Almost nobody plays DM. Its rooms are often empty and it is easier you see a shiny pokémon than you see people playing it.
I think the main reason for that is a lack of prizes. Every other modes have a real earning besides BattlePoints.
The newest mode Coliseum, is far more intersting to DM. Every week we have a brand new one, with good prizes and variety in gameplay.
The DM was created to be a mode like a mini - tournament, why not give it a due prize ? Like 1 credit per 2 DM won (it's not that much) and include the winners of DM in a Lottery of rares, uncommons and commons ? Or at least include the winners in a lottery of daily tournament ?