There is only so much time in a day. Often singing in choir conflicts with playing sports because you have concerts and games on the same nights, so you have to make a choice. There are also schedule pressures - if you are going to get into college you nearly have to take math, English, science, and foreign language classes beyond what your school demands and that forces hard choices if there even is a class period free (don't forget you might be taking band to take up that space)
Finally, there are a lot of bad teachers. They are so interested in winning competition and teaching perfection - but for most music will never be anything other than a fun hobby and so they are getting the wrong teaching which turns many students off. I've seen a lot of award winning school choirs, and the next town over with the same number of students has twice the students in choir despite not winning awards - communities need to pick and often don't realize this.
In my secular chorus, we may have two main performances per season, but we rehearsed together for 2 hours every single week for months. We purchased polo shirts, and there was a dress code. Our dues covered operating costs and sheet music. Being a civic group for casual singers, our costs were kept low, but many choirs travel, double down on the costumes, and many people find it requires a high level of dedication, free time, and independent wealth. It is no coincidence that many members are retirees!
In church, a lector could prepare for 30 minutes and have a 5 minute speaking part at the Mass. The ushers and EMHCs also have part-time gigs. While altar servers are on duty for the entire service, they do not need to rehearse every single week. A church choir may serve for one or more weekend services, plus the 90-120 minute weekly rehearsals, and that's not counting holy days, Easter, Christmas. If you take a role as cantor, director, or piano/organ, expect to become indispensable! Some families just found it easier to join en masse so they could stay together.
Some chorus members are also secretly voice coaches, so if you protest "but I can't sing tenor" they may lovingly tackle you and sell you a package of private lessons.
I found it difficult to serve in any other ministry alongside choir, and you may find it difficult to hold down a job and/or family alongside a secular chorus role. As I said, high demand/great rewards.
N=1 and not singing in a choir, I could probably sing tenor parts if I trained, but to me bass feels more satisfying, doubly so going low with throat singing.
In my part of the world, it's definitely declining church attendance. If you don't have a huge population of young boys and girls trained in those choirs, and instead the population were self-selecting into voice training, I would definitely expect a serious sex imbalance.
I've found some excellent vocal groups from parts of Asia not known for being especially Christian, but it seems that choral/a cappella music is very connected to Christianity there too.
I believe church attendance is getting more gendered too, with a shortage of men.
I'm in Houston. I have seen nice a cappella from the Philippines but that must not be what you have in mind, large Christian population there. Where were yours from? From what I've seen of African church singing, they were always mostly female.
As a professional-level baritone who has sung tenor parts quite a lot, there is a shortage of every low voice type (directors are often conflicted when I make the offer to sing tenor). People who can produce a chorally-acceptable A or Bb are in the shortest supply, though. It's getting worse as the amateur singing circuit gets smaller and the gender ratio gets more skewed.
Amateur-level choirs tend to have a lot more basses than tenors because it is easier to sing bass without effort spent on vocal training.
I am an amateur baritone, in school I was used for tenor parts because of course there was a shortage and I had good enough technique that I could sing tenor parts, if not well.
Now, I sing second bass for a men's choir, because that was what they were missing. I think all not in-the-middle voices are scarce.
Tenor parts are more difficult, technically speaking, and voices capable of the tenor range are rarer. So any given man joining a choir can more likely manage the bass range, and if they can, they can almost certainly manage the bass parts.
FTA:
> When men do join singing groups, they often avoid the tenor section. The tenor voice is “a cultivated sound”, says John Potter, author of a book on the subject. A man with no vocal training is more likely to have the range of a baritone (a high bass). It does not help that the tenor voice is associated with operatic stars such as Luciano Pavarotti, who could powerfully sing high notes that no amateur can easily reach. And the tenor line in classical choral music can be difficult, with many unexpected notes and alarming leaps.
surely a real bass is rarer? I just assume, as someone completely musically inept, based on listening to the vocal groups on the radio, that a bass contributes less, and can be omitted more easily?
People at the extreme ends of the spectrum of range are rarer and people in the middle of the range are more common. As it stands, choral bass parts fit better into untrained voices than choral tenor parts. A typical baritone (middle range male voice) can sing choral bass parts well enough, but will find tenor parts relatively strenuous.
I know many women who admit they "fall in love" anytime they hear a low bass. They might marry a tenor and never cheat on them, but every time their hear a low bass their heart flutters. Men know/see this and so tenors become less interested since their higher voices don't get the women (there are plenty of other ways they have).
Anecdotal I guess, but when I was in a high school choir, I loathed that my teacher assigned me to the tenor section. It did not fit with the image of myself that the high school version of me held in my head; "a man should be a baritone or bass after puberty!"
I liked choir and stayed in it for all four years, but I was never particularly good at it so what the hell did I know anyway.
"average bass steals all the love interests" factoid actually just statistical error. average bass steals 0 love interests per year. John Tomlinson, who steals 10,000 paramours per year, is an outlier and should not have been counted.
It's not clear what the evidence is that the problem is getting worse though? Or why it would be?