Although this may not answer your question, it may help to shed a little light on the subject. Other Christians have claimed that Catholics invented Baptism by sprinkling in the middle ages or so. But the Didache, which is the oldest known Liturgical guide (around 70-95 AD), lists Baptism by Immersion, and Sprinkling as valid types.
From “Apologetics and Catholic Doctrine” by Michael Sheehan:
It is not true that full immersion remained common even in the early Church: of the dozen or more 3rd and 4th century baptisteries excavated in Greece, only two have fonts a metre or so deep, and most are under 50cm, i.e., knee-deep if full. The same patterrn is true of baptisteries found in Syria, Palestine, Egypt and N. Africa. A fresco from the first half of the 3rd century in the Catacombs of St. Callistrus, Rome, shows a baptism being performed in water a few inches deep. From all these it is clear that an adult candidate stood in a shallow pool, and some water was gathered from it and poured over his head.
Sources: J.G. Davies, The Architectural Setting of Baptism, Barrie and Rockliff, London 1962; S.A. Stauffer, On the Baptismal Fonts: Ancient and Modern, Grove Books, Nottingham 1994; Bellarmino Bagatti OFM, The Church from the Circumcision: History and Archaeology of the Judaeo-Christians, Franciscan Printing Press, Jerusalem 1984, p. 245; Id., The Church from the Gentiles in Palestine: History and Archaeology, Francisc. P. Press, Jerusalem 1984, pp. 301-8.
Again, I don’t know why the Church does not do it, or even if it insists that we don’t do it. I’m under the impression that if you specifically ask the priest for immersion, and the ability to immerse is available, then they will do it. But I’m not really sure about that.
Take Care and God Bless!