Once we entered into later periods, particularly starting in the Classical era, the alto line in choral music became more like what seems "an after thought" for many composers. Creating parts that are terribly boring, or conversely, alto lines that "fill in the gaps" of the chordal and/or rhythmic structure without having much of a line of their own, that verge on the unmusical. The "catch-all" line.
You know the joke: "Altos are just sopranos who can read."
Yes, there are a few modern composers who write real lines for us, but altos have never really regained their position in choral music in any kind of widespread way.
However, now, alto sections are littered with the remains of sopranos who have been "demoted" because they have lost their high notes, or who never trained to have any.
Yes. I said "demoted". That is the word I have heard used time and time again in regard to this common practise. It's negative propaganda. As if being an alto is somehow, a lesser position, rather than a voice type. And that negative propaganda has certainly led to that widespread mindset and perpetuation of "after thought" alto lines in compositions. And that propaganda has certainly effected the numbers of altos who are putting it out there.