Maybe it’s the influx of ex-Catholics into Protestant churches who brought their hymnody ideas (or lack of same) with them?

Hmmm - maybe its a subversive plot to destroy Protestant hymn singing.
Although I do agree that schools do cut the arts first…
Phemie is correct about this.
Many of the evangelical and non-denominational Protestant churches have gotten rid of hymnals, organs, and even pianos, and have gone entirely to using Praise and Worship songs projected on an overhead, or not projected at all, but merely led by a Praise and Worship band. The congregation is expected to pick up the lyrics by just listening and learning.
This was/is very frustrating to those of us who read music.
I have a terrible time singing by ear. This is one of the many reasons why I do not have good results from trying to learn any kind of chant from merely hearing it, even dozens of times. I have to have music (preferably modern notation–the neumes are rather challenging!). I wish I were built differently so that I could sing or play by ear, but I am what I am.
It was also frustrating to those of us who were raising children. When my children were young, I always insisted that they hold the hymnal or look on with me or my husband as we held the hymnals, and I would point to the NOTES (not the words). The girls honestly did learn quite a bit about reading music when I did this, and by the time they were in 6th grade, they could sight read well-enough to participate in our city’s annual presentation of Handel’s Messiah (not an easy sight-reading task).
(Note–my girls attended a private school and the music education was excellent. All children were taught from age 3 how to read music and how to sing correctly. The teachers used the solfege method.)
However, while they were growing up and we were attending the churches that used overheads and no hymnals, the girls didn’t get the benefits of singing hymns with music notes. (My daughters hated Praise and Worship, BTW. They called it “Boomer Rock.”)
My husband used to subscribe to several evangelical music magazines, and once he showed me an article extolling the value of getting rid of the organ in all worship services and using only a keyboard or a praise band. Again, this is musical disaster for young ones, because the organ is an excellent instrument to learn to sing with because it sustains the tones so beautifully, and because it is similiar to a human voice. The praise band, especially one played by amateurs, tends to “muddy up” the melody and make it very hard for children (and many adults) to hear the melody and sing on pitch.
No wonder we see so many tone-deaf CHRISTIAN children.
Thankfully many evangelical churches still offer a “traditional” worship service, too, where the old hymnals with the 4-part scores are still used!
And I agree–Catholic hymnals and missallettes SHOULD include the 4-part harmony. But would you believe that there are people (I’ve met up with them on this forum) who believe that 4-part harmony is “Protestant” and therefore shouldn’t be done in the Mass?!! Ay yi yi. I fear that Catholics get the Mass music that they deserve.