Standing during prayer has been a Christian tradition since the very beginning of the Church. This can clearly be seen by looking at various images drawn in the catacombs during the first few centuries of Christianity, which depict men and women standing in prayer.
This was also a common way of praying for ancient Hebrews, and many early Christians adopted such modes of prayer.
Sign of respect and adoration
Naturally, standing became a typical posture of prayer during the celebration of Mass. While it's true that kneeling was also part of liturgical prayer, standing was highly favored and seen as a sign of respect and adoration. The Council of Nicea in 325 even went so far as to ban the faithful from kneeling at the Sunday liturgy, so as to promote uniformity.
Since there are some who kneel on Sunday and during the season of Pentecost, this holy synod decrees that, so that the same observances may be maintained in every diocese, one should offer one’s prayers to the Lord standing.
For this reason, Catholics (and Orthodox Christians) who maintain the Eastern traditions of the Christian faith always stand during the Divine Liturgy. It is not customary to kneel during the consecration of the Eucharist because in the Byzantine culture, kneeling was associated with penance, not adoration.
Here is one simple description of the practice from a Byzantine church.
We are standing with our Resurrected and Glorified Lord and this is our proper liturgical sign of being united with Christ as one body. There is no disrespect in standing. Kneeling is considered penitential. We only kneel at certain services during Lent, such as during our Liturgies of the Presanctified Gifts.
While this practice may sound strange to Roman Catholics, it is completely acceptable and is in accord with the culture of Eastern Catholicism. Standing and bowing during the Divine Liturgy is as reverent as kneeling is in a Roman Rite Church.
Later on the Roman Rite found kneeling to be an appropriate sign of adoration and did not associate it with a penitential attitude. Kneeling and standing during Mass are smaller "traditions," that can be changed according to the current culture and are not essential to the faith.
The Church allows for many traditions to be practiced within the liturgy, and each highlights the beauty of culture and its ability to communicate the same truths through different signs and symbols.
