Looking at old language presentations of programming languages that never managed to catch on I am often very interested in figuring out just why it failed.

Why is this useful?

I think it's useful for language designers to consider why some things fail and why some things succeed. In the end a language is serving some intended group of users [1], so ask the question "why didn't it succeed in doing that?"

I believe it's an important thing to ask, because the answer often isn't that "the language was bad". It often wasn't a bad language, but there was still something it failed to do which prevented people from using it.

It also implies that in order to actually serve a group of users (the presumed goal of a language) we do not only need to create a good language, but also a language which succeeds in reaching the users.

In order to succeed at language design we must not only make sure that the language is good, but also ensure that there is a way for the intended users to make use of it.

Why do languages fail?

The obvious and most common way a language can fail is by never being completed. It doesn't matter how good the features are if the language can't be implemented.

The second big thing I see is the "build it and they will come" thinking. That is, the idea that all you need to do is to write a sufficiently good language and then somehow that should be enough to make the language universally adopted by everyone. Unfortunately reality does not work like that.

Looking at successful languages, there is no real common pattern. Corporate backing helps, but isn't a guarantee. Lots of initial interest is good, but doesn't mean it will be a success etc.

But while it's difficult to clearly make out the road to success, we can still study other languages for clues on how to avoid the paths leading to failure.

In the end the failure to understand why languages fail is the biggest reason why languages fail.

[1] Zig has "together we serve the users" in its mission statement – addressing exactly this.