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.

On the point of there always being a shortage of tenors, these charts are interesting: https://www.researchgate.net/figure/Frequency-distribution-o...

The shape of the distribution has a much longer tail for women than men, on the high pitch side.

"Or why it would be?"

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.

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.

I suspect it's become a problem the moment it became a primarily amateur endeavor. There's enough tenors when you can pay.

Declining popularity among men

i think it's very hard to sing all the way up to the high C5 as a male, this doesn't surprise me at all