I'm not saying popularity = good, what I'm saying is that most people aren't smart or patient enough to use something like APL. Most people have a limit to the amount of density they're willing to deal with.
Well, I can only hypothesize, but APL is a lot like math, visually anyway. Most people are very intimidated by math.
Math tends to be written for subject matter experts. If I'm reading a paper about graphics, I know that θ is "theta" and likely means "angle in radians". If I'm not a subject matter expert, it means nothing. Of course they can write at the top "let θ = light incident angle in radians", but I still need to hold that definition in my head while I'm trying to understand the rest of it. If I simultaneously need to remember ⍵ and ε and Δ, well, I can do that if I've seen those symbols used in similar ways previously, but it's certainly going to increase the amount of things I need to hold in my head if it's something novel. And now I have to memorize what those things mean, and I suck at memorizing personally.
In contrast, most mainstream programming languages you'd just write "angle_rads" or some variant. It's a little longer, but I don't need to remember nearly as much because it's just plain english formatted in a way that at least resembles sentences I might read. I just think APL is too unfamiliar for it to be easy to learn, and so the only people that are going to learn it are those that are very intellectually curious. That's a good thing, but if you're talking popularity? Well, I think we've seen it, and it's not just path dependence.
