Because the Catholic Church had "the keys." They locked up the "mysteries of scripture" from the laity. The Catholic church clearly did not use the keys to unlock the kingdom of God, but to shut it up.
(Matthew 23:13 KJV) But woe unto you, scribes and Pharisees, hypocrites! for ye shut up the kingdom of heaven against men: for ye neither go in yourselves, neither suffer ye them that are entering to go in.
It literally took a reformation to unlock the kingdom and to return the scriptures to the laity. They had removed the wine from laity in the sacraments and they had removed the scripture from the laity in their worship.
Thank God the gates of hell did not prevail against the true church and God sent Luther and Calvin and Zwingi and Wycliffe and Tyndale and other brave and noble Christians to rescue the scriptures from the Nicolaitans and present it once again as God's gift to the Body of Christ, the community of true believers in Christ.
O how love I thy law! it is my meditation all the day. Thou through thy commandments hast made me wiser than mine enemies: for they are ever with me. I have more understanding than all my teachers: for thy testimonies are my meditation. I understand more than the ancients, because I keep thy precepts. I have refrained my feet from every evil way, that I might keep thy word. I have not departed from thy judgments: for thou hast taught me. How sweet are thy words unto my taste! yea, sweeter than honey to my mouth! Through thy precepts I get understanding: therefore I hate every false way. Thy word is a lamp unto my feet, and a light unto my path. I have sworn, and I will perform it, that I will keep thy righteous judgments. (Psalms 119:97-106 KJV)