Why not? Our priest put the blessed sacrament out in a monstrance.
Because the instructions for the reservation of the Blessed Sacrament on Holy Thursday say so. From
Paschale Solemnitatis, nn. 54-55:
IV. Holy Thursday Evening Mass of the Lord’s Supper
After the postcommunion prayer, the procession forms with the crossbearer at its head. The Blessed Sacrament, accompanied by lighted candles and incense, is carried through the church to the place of reservation, to the singing of the hymn “Pange lingua” or some other eucharistic song. This rite of transfer of the Blessed Sacrament may not be carried out if the liturgy of the Lord’s passion will not be celebrated in that same church on the following day.
The Blessed Sacrament should be reserved in a closed tabernacle or pyx. Under no circumstances may it be exposed in a monstrance.
The place where the tabernacle or pyx is situated must not be made to resemble a tomb, and the expression tomb is to be avoided: for the chapel of repose is not prepared so as to represent the Lord’s burial but for the custody of the eucharistic bread that will be distributed in communion on Good Friday.
(The USCCB
follows the same rules: “The Blessed Sacrament should be conserved in a closed tabernacle and should not be exposed in a monstrance.”)
Now,
why is this the case? Well, I’ve never heard an explanation, I’ve never been educated as to why, but I could guess. Because this is the time during which Jesus has gone out to Gethsemane with his apostles, and is praying in seclusion and agony. He is sequestered, almost in hiding. And, furthermore, he is not displaying his glory now: he is entering into his most unglorious moments (from our perspective).