because although it can and does work, cc does have it's downfall, and it is not quite as good as a dsb, cc will work, but til you buy cc and a ugf, you have spent more than you would on a dsb, and a dsb is less work, more efficient and is not the potential detritus trap that a dsb is(meaning it traps the garbage which then turns into nitrat and does not expend the nitrate, a dsb does all 3 and is much less work), trust me i had both at one time, no more, now i have only a dsb with lr, NO FILTERS WHATSOEVER, most people can't even imagine getting the water quality i have with a dsb(unless you have a skimmer and clean the gravel), the only thing i have (with no skimmer or filters) is a slight trace of nitrAtes, 0 ammonia, 0 nitrite, 0 phosphate, and teh other parameters are perfect, with cc and a ugf, i did much more work, and could not even come close to 0 nirate