It's probably due to costs. Hildy questlines are pretty expensive to design from an animation/camera angle standpoint.

Though Relic weapons have always been a staple - yet not a mandatory quest to do. I don't see a problem to combine both. As everyone knows... relics are pretty much optional. You just basically grab it for glamour. It will take quite a while before it can even become the highest stats for a weapon - generally near end-expansion, so it's generally faster to grab a raid-weapon /tomestone gear weapon instead.

Plus, it's a huge timesink to spend. Having the Hildebrand questline tied into this... doesn't seem much of a difference? You're already spending a huge timesink to get the weapon anyway. Seeing that you can skip hildebrand cutscenes if you don't like them, it's not much of a difference.