it is a "little" harder to decap older dark comb. There is also the "thought" of all those varroa defecating in the cells, and the larvae spinning their coccoons etc... some people dont like such things........... so dont tell them.
next, as mentioned, the absorption of undesirable chemicals.. Your bees DO bring it in, unless you live on an island all alone, and own the island so you KNOW no chems are being used.
Wax absorbs that cocktail of miscellaneous stuff brought it.. from peoples gardens, from their flower beds, from nurseries, from commercial crops, even from Yards sprayed to kill those beautiful dandelions. Everywhere you look, there are chems being used, fungicides, pesticides, herbicides, etc, etc... ALL of those things are present in old dark comb..
Some people dont like the thought of eating honey from dark comb..