One result of this has been considerable confusion about Advent (already sufficiently confused by its double focus on the two comings of Christ).
At the time of the Reformation the traditional readings were more or less taken over by the Reformers, who also adapted one of the Collects and added three new ones. The Advent Collects generally follow the theme of the Epistle for the Day. A study of the Gospel readings and Collects gives an idea of the focus of Advent in the Anglican Church. In general this is directed to the ministry of Christ as King in his first coming and (to a lesser extent) his return as judge in his second coming.
The reason that the announcement and birth stories are not included is because these are appropriate for the Christmas period that follows. Another reason is that the coming of the King is a bigger story than just the birth. It is noteworthy that two of the four gospels do not mention the birth but begin the story with John the Baptist.
Here are the Collects and Gospel readings as specified in the Book of Common Prayer of 1662. The Gospel readings from the modern Australian Lectionary are in brackets.
The first Sunday in Advent
Gospel: Matthew 21.1-13 – The triumphal entry and cleansing of the Temple [Luke 21.25-38]
Collect: Almighty God, give us grace to cast away the works of darkness, and put on the armour of light, now in the time of this mortal life in which your Son Jesus Christ came to visit us in great humility; that in the last day, when he shall come again in his glorious majesty to judge both the living and the dead, we may rise to the life immortal; through him who lives and reigns with you and the Holy Spirit, one God, now and for ever. Amen. (Epistle: Romans 13.8-14)
The second Sunday in Advent
Gospel: Luke 21.25-33 – signs of the coming of the Son of Man [Luke 3.1-6]
Collect: Blessed Lord, who hast caused all holy Scriptures to be written for our learning; Grant that we may in such wise hear them, read, mark, learn, and inwardly digest them, that by patience and comfort of thy holy Word, we may embrace, and ever hold fast, the blessed hope of everlasting life, which thou hast given us in our Saviour Jesus Christ. Amen. (Epistle: Romans 15.4-13)
The third Sunday in Advent
Gospel: Matthew 11.2-10 – Jesus reply to John the Baptist’s inquiry as to whether Jesus was the one they were expecting. Answered with references to Old Testament prophecies. [Luke 3.7-18]
Collect: O LORD Jesu Christ, who at thy first coming didst send thy messenger to prepare thy way before thee; Grant that the ministers and stewards of thy mysteries may likewise so prepare and make ready thy way, by turning the hearts of the disobedient to the wisdom of the just, that at thy second coming to judge the world we may be found an acceptable people in thy sight, who livest and reignest with the Father and the Holy Spirit ever, one God, world without end. Amen. (Epistle: 1 Cor 4.1-5)
The fourth Sunday in Advent
Gospel: John 1.19-28 – John’s witness to Jesus as the Lamb of God who takes away the sin of the world. [Luke 1.39-45]
Collect: O LORD, raise up (we pray thee) thy power, and come among us, and with great might succour us; that whereas, through our sins and wickedness, we are sore let and hindered in running the race that is set before us, thy bountiful grace and mercy may speedily help and deliver us; through the satisfaction of thy Son our Lord, to whom with thee and the Holy Ghost be honour and glory, world without end. Amen. (Epistle: Phil 4.4-7)