Here's one of several reasons:
What in the early Church and during the Middle Ages determined the position of the altar was that it faced East. To quote St Augustine: "When we rise to pray, we turn East, where heaven begins. And we do this not because God is there, as if He had moved away from the other direction on earth..., but rather to help us remember to turn our mind towards a higher order, that is, to God." This quotation shows that the Christians of those early days, after listening to the homily, would rise for the prayer which followed, and turn towards the East. St Augustine always refers to this turning to the East in prayer at the end of his homilies, using a set formula: Conversi ad Dominum ("turn to face the Lord").In "The Reform of the Roman Liturgy: Its Problems and Background" (p. 80) by Msgr Klaus Gamber (Una Voce Press, 1993).