Thurs Sotah 38

We only say the Shem Hameforash by Birchas Kohanim in the Bais Hamikdash.

Tosfos bring Sifri that the reason is you can only say it where the Schina is exposed. As it says"the place where I come" refers to where He Himself comes, where the Schina is exposed.

Therefore Tosfos says, this is the reason they started refraining saying Birchas Hakohanim with the Shem Hamifurash after the death of Shimon Hatzadik. Afterwords, they didn't merit much exposed Shchina.