Maybe it is. Maybe not. But there’s nothing theological in the Church’s reasoning in favor of organ and chant.
The pipe organ (and note that the moniker “King of instruments” does not apply to electronic or other organs) does have some practical advantages. It enables one person to fulfill all the functions that the best accompaniment can provide:
1. It can support the singers (choir and especially congregation) at least an octave beneath the voices, so as to provide a tonal and psychological foundation for better singing.
2. It can surround the singers in their singing range and even blend their voices because the organ, like the human voice, is a wind instrument.
3. With an advanced player, the organ can cue changes for the singers: starting and stopping, modulation, etc..
There’s no question the organ is versatile. But a well-honed musical ensemble can provide all these things: support, surround, cues, and the like. It can be harder to get several people working together than to find one person who can do it all. We also have the “political” consideration of the organist: one person is easier to control than a small orchestra.
The Church’s argument for chant is simple enough. Chant permits the music to adapt entirely to the liturgical text. Hence, no need for changing the words to fit a meter or rhyming scheme. This, too, may be somewhat political, but that’s fodder for another post.