Because it stops being an endgame activity if teams of low level blueberries can casually stride in like it's no big deal.
That defeats the purpose of Trials.
No LL also means that everyone can use retired weapons, leading to a stale meta.
The retired weapons bit is a good point I hadn't thought off but by the looks of it there will be plenty of reskins that do the same thing that people can grind for again.
I keep hearing this reasoning that it's an end game activity, but so what? If the blueberries you mention can compete than all the more power to them (no pun intended). It's not like we have matchmaking so they have to make teams regardless.
I can understand the power mechanic in pve may be needed as an artificial increase to difficulty but pvp is different. Power level is not needed as the difficulty is organic and unpredictable.