Balance. The developers explicitly said many times that relics would remain the "best" weapons. Obviously the developers don't have the same conception of best as players, but they are smart enough to realize they couldn't keep relics at 75.
Moreover none of the other gear is tied to Abyssea. No other gear has trials tied to Abyssea only. Empyreans are the outlier. Given that the theme of empyreans was to restore weapons of the Champions of Abyssea
Additionally they seem to know that they overpowered the empyreans, they would likely still destroy 99 weapons if kept at 90 from the aftermath alone.
In any case the explicit word of the developers is to bring relics up to or above empyrean. Now that isn't going to happen for some, like knuckles, where the ws just won't permit balance, but that its not the case with the daggers where the ws are not very different.