[dangus]

The reason may be quite simple:

At Catholic masses, there are three readings. The first is from a New testament letter. The second is from the Acts of the Apostles, Revelations, or the Old Testament. The third is the gospel. (There is actually a fourth, the psalm, between the “first reading” and “the second reading.”)

The psalm and the first two readings may be read by a layperson, from a parapet or lectern which is not considered part of the sanctuary. The gospel may be read only by a priest.

[kosta50]

The reading of the Old Testament in the Cathlolic Mass is, I believe, a latter-day (a post Vatican II experiment) addition. The Orthodox never read from the Old Testament during the divine liturgy, except during Lent. The liturgical text itself (St. Joh Chrysostom's liturgy going back 1,600 years) includes 2 Psalms, which are sung by the choir and laity as antiphonal responses.

The Reading of the Apostle (Epistles) is from the lectionary by a lay person. In those churches where there are pews, the congregation sits during the readings of Pauline letters. I want to emphasize that the Apostle is a separate book and is never carried to the altar, and neither is the Old Testament.

Everyone stands when the priest/deacon sings/reads the Gospels. This is in keeping with the Judaic roots of liturgical practice of standing only when the Torah is read.

The Old Testament readings, including and in fact predominantly, the Psalms, is done during the evening services (vespers). The only time when the OT is read (from the beginning until the end) is during the forty days of Great and Holy Lent.