Because it does look good, always did. There was no issue with that. There does not need to be any rationale to it, after all, its aesthetical thing, thats not always exact science. Either you have feel for it, or you dont. If you look at the away jersey from last year with black bants etc... and current blue abomination, and dont see how the former is right, while the latter is wrong, dont know what to tell you.
If you however insist on some rationale anyway, lets just go with continuity. It was there for quarter of century, its part of the identity, its one of the things that differentiates. Imagine Devils getting rid of black from their jersey and run with just red/white. But they wont, you know why? Because they are not f***ing Red Wings! Look how swiftly Carolina returned to black on their jerseys after short period with pure red/white ones... cause they looked like Red Wings! Do you want to Avs be another Montreal/Columbus/Rangers/Washington/whoever else?