There are two reasons that we do not refer to God as Yahweh:
Since the name/word was only pronounced once a year by the High Priest its true pronunciation has been lost to history.
Jewish tradition has avoided pronouncing or even writing the name of God out of holy respect and awe. Out of respect for that tradition the Church has avoided using the name in liturgy and has even asked that music not include it: No ‘Yahweh’ in songs, prayers at Catholic Masses, Vatican rules
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