An Elemental's AoE on the souls is entirely position reliant whilst the DK can adjust completely, not to mention that, at low gear levels, the extreme earthquake damage shown wasn't as reliable or consistent as having DKs with Reaper's Harvest available to tie in with a warrior rending/executing low HP adds.
Talking about Nihilum who killed it 3 months ago has no relevance on a parse done 2-3 weeks ago because the game is completely different on farm vs. on progress, output is only half the story, consistency is what decides your setup. Ele is bad at every other aspect of the fight compared to other ranged, i.e. a Hunter. Taking a DK fills up a melee spot to spread out gear and keep the raid balanced, whilst doing the same job, being better downstairs, and doing more damage consistently to the targets without said padding aspects. The only melee you'd take instead of a DK would be purely boss damage padding or alternatives that are stronger at adds, which DKs are very competent at.