There is precious little on the internet about the Catholic church’s decision to withhold the cup, and a lot more of them patting themselves on the back for allowing it now.
Seems like they aren’t particularly anxious for anyone to know.
Perhaps you are looking in the wrong place. You should be looking in the Bible.
"I am the living bread that came down out of heaven; if anyone eats of this bread, he will live forever; and the bread also which I will give for the life of the world is My flesh." - John 6:51
The Church teaches that Christ is contained whole and entire under each species; so that whoever communicates under the form of bread or of wine receives not a mutilated Sacrament or a divided Savior, but shares in the whole Sacrament as fully as if he participated in both forms.
From this passage it is evident that whoever partakes of the form of bread partakes of the living flesh of Jesus Christ, which is inseparable from His blood, and which, being now in a glorious state, cannot be divided;
"For we know that since Christ was raised from the dead, he cannot die again; death no longer has mastery over him. Romans 6:9
With the exception of the last supper and His agony in the garden, Jesus makes no reference to the sacramental cup, but only to the Eucharistic bread, to which He ascribes all the efficacy which is attached to communion under both kinds, these being union with Him, spiritual life, eternal salvation.
Similarly, St. Paul, in his letters to the Corinthians, said:
"Whosoever shall eat this bread, or drink the chalice of the Lord unworthily, shall be guilty of the body and of the blood of the Lord." 1 Corinthians 11:27
Peace be with you