If Jehovah is God’s name, why isn't it used in church?

Your protasis, your supposition, is false. Jehovah is not God’s name. It is a sixteenth century English mistaken rendering of the Jewish tetragrammaton yodh-he-waw-he, usually given in Latin script as YHWH. Since the Jews do not pronounce the sacred Name, but replace it in reading with “Adonai” = my Lord, or “ha-Shem” = the Name, we do not actually know for certain how it was pronounced. Only the High Priest uttered it on Yom Kippur. Yahweh is the common consensus. It means “He Who Is”, “He Who causes to exist.” Out of respect for Jewish sensibilities, Pope Benedict XVI asked Catholics not to use YHWH in the Liturgy, but to say “the LORD” instead. For Christians of course the Sacred Name is Jesus (Hebrew Yeshua), the Name above all other names, at which every knee shall bow, and every tongue confess, that Jesus Christ is Lord.