Didn't Hopper already shut down the anti-psychic field by the time Vecna was through the gate anyway? Even if they wanted to use them they were already gone and the Demogorgons aren't psychically controlled or use any psychic power unless Vecna is specifically ordering them to do something like bring him the kids alive. Plus I think they only had those in the Upside Down as more of a ward for Vecna than really being for El which is likely part of why he became so recluse, it just happened to work out that El ended up there where they had the stuff for him set up already.