Judging from most of the posts in here and in previous threads complaining about it, the reasons really come out to a pretty thinly veiled complaint that too many people have a relic.
That the value only comes form fewer people owning one, and too many people earning it too fast somehow cheapens your experience and the worth of your item.
It's complete nonsense to me, you aren't "earning" anything more by doing 11 runs of syrcus tower in DF than the person doing hunt trains to get the tomes for the new relics.
60 runs of alexander isn't somehow more compelling than 2 days of roulettes or somehow takes more skill or effort.

It's that when the grind isn't abysmally boring and frustrating to do, more people do it and your shiny weapons aren't special anymore.
They never were special though, they're just a glowy weapon that looks kinda cool and is BiS for ultimate.