I'd say that's because a lot of modern Black metal that people on this board fellate, still retains some of the conviction, inspiration, and dark vibe which fueled the works of the old masters, while the majority of popular death metal mostly went to emulate the works of more accessible At the Gates and Suffocation records. the two camps trying to out-maneuver each other in technicality, brutality and writing more catchy material. And that's not what most old-school veterans are looking for.
Of course,there's contemporary old-school death metal, but I find death metal to be a somewhat more restrictive genre,so when writing new material in the old vein, it's more difficult to reinvent yourself. It's difficult to capture that dark feeling without sounding artificial, stepping too much on your own foot warts and on the feet of those who came before you.
Just look at all the reunion albums of old bands that tried to do it. They were labeled sellout on release. There maybe only a few of the devoted old-timers who would be curious to hear a squeaky modern sounding Demigod. The majority of fans wants them to recapture the darkness of their past. They want Slumber of Sullen Eyes, part 2, a direct continuation of their magnum opus. Nostalgia is a bitch, mate.
Black metal is dark by virtue of the specific techniques it employs, while necrotic death metal of the early 90s gets its dark vibe from the raw, unpolished sound which is often due to constraints in production budget.
