There are many reason here are a few
1. Carbon is suspected to either cause or worsen HITH disease in cichlids
2. Most people do not change their carbon often enough. when carbon is in the water for longer than the recommended time it will start to leech any impurities it has removed back into the water.
3.Carbon can remove ammonia too well which can starve your beneficial bacteria hence destroying your biological filtration.
There are pretty much only 2 reasons to use carbon
1. TO remove medication
2. To remove a foul odor from the water that water changes do not seem to affect.
Basically If you have a clean tank with healthy fish and good biological and mechanical filtration Carbon is not needed and in some cases can become harmful.
I am sure I left something out but you get the point.