The reason I have seen often and that makes sense to me is that in the early days Black Metal had the "Black Metal sound" because of the feelings and ideas that the musicians were trying to portray. the newer more modern bands have the "Black metal sound" because that is what black Metal is supposed to sound like. Instead of creating a unique, powerful, sound with a meaning, they simply make music that sounds like Black Metal.
Make sense?
I suppose it is the equivalent of death metal fans saying DM today has the br00tality and technicality aspect of DM but sorely lacks the passion, the energy and the memorablity of old school DM