> When I ask those questions I am in no way trying to discredit new languages and their usefulness, I am just young, naive, not very smart and trying to get and idea of how the real world of programming and computer works.

Welcome to the war.

Please don't hold any hard feelings for the community if you get flagged or downvoted to hell. People who will do this to you are generally smart, sympathetic and considerate individuals who just were on the frontlines for much too long. Being cold-hearted and eliminating every threat swiftly, no matter how innocent it seems, is the only way of preserving one's sanity here.

I'm a PLT and Type Theory enthusiast, although I lack any formal education in this direction. I try to follow new research and I'm constantly learning new things (like the ones from the '60 which were then forgotten) and really new things (original research happening now which acknowledges what was done in the field already). I graduated (last year) from just learning new languages and I'm writing my toy languages (thanks to Racket's being an absolutely wonderful framework to do so), but I still learn every single language that seems interesting. This includes both nearly-mainstream languages like Erlang and the ancient, largely forgotten like Prolog, APL and Forth (which you should include in your list next to C, Smalltalk and Lisp).

I'm fascinated by the notion of computation, of how we can encode computation, how we can reason about computation and how we can transform computation to preserve its semantics. I'm fascinated by language design: what features a particular language has and what it omits, I'm always trying to discover what kind of turtle (and if really all the way down) a language is built upon. I'm feeling happy and safe reading papers from Racket and Haskell people, it feels like I'm reading a suspenseful novel in a quiet library somewhere.

Then I go to StackOverflow or here and the reality hits: screaming, shooting, blood and intestines everywhere, people fighting for their salaries and self-respect, so ultimately for their lives.

You'll hear about technicalities from other people here: type systems, concurrency primitives, memory safety and direct memory access, static vs. dynamic (not only typing), syntactic support for common idioms, having (or not) a built in support for certain concepts (like inheritance or composition). I'm not going to tell you about all this. I'd love to, and I really like the topic, but I feel that you wouldn't benefit from it nearly as much as from the other half of the story.

You see, programming languages are tools which people make for people to use. Not only that - both the makers and consumers do what they do to feed their families. I recently saw a Byte magazine from 1980 (IIRC) where I saw an ad of TinyPASCAL, which promised 4x increase in speed over the equivalent code in Basic. It came with some additional libraries (and it was available for a couple of different machines) and cost $8. There was another ad, which claimed that you won't ever need another Fortran after you buy the one being advertised, because it was fast and had additional libraries, for example (IIRC) for calculating log (or lg). It was some $15, I think. Not having lived then I miss a lot of context, but what I see here is that people were using programming languages to make money for quite a long time.

This is not a problem in itself. The problem is the nature of our industry, which is for the most part impossible to measure or experiment with. When have you last heard about double-blind (how would that even look like...) experiment of building the same large corporate system 5 times with different tools and simultaneously? I didn't. And that's not all. We are certain about some things, because the mathematicians discovered some very clever proofs of these things. But they are rare, few and far between. For my favourite example: what "readability" even is? People fight to their last breath and last shred of dignity for their particular take on readability, yet we don't have a slightest idea what the hell readability is, let alone how it impacts us. It's the same, just many times worse, with other features, like famous allowing assignment in conditionals, or preferring mutability over immutability, or providing pointers or not and so on. We know for sure that, if the language is reducible to a very few operations which form one of the basic models of computation, that it's able to express everything expressible in every other language. That's a baseline and it's basically useless, because there are real differences between how good are different languages as a tools and we have no idea at all what makes the difference. We have lots and lots of anecdotes, though.

All this - people wanting better tools and people getting used to their tools, people designing new tools and people marketing the tools they make as better, and having no meaningful way of defining what "better" even means here, but having a vague feeling that how good the tool is directly impacts your performance and your pay leads to the current situation. People have their beliefs, and there are people - some sincere, some not so much - who profit from their beliefs. Languages are being viewed as tools for writing software and for generating revenue... both by corporations and individuals. All programmers make decisions about which philosophy, which belief system to buy into and they all know that this decision is an important one. For companies it - having a language with large following - can make a whole difference between winning and loosing on the market. Similarly for individuals, belonging to a particular tribe makes them feel safer, they can more easily ask for help, they can find jobs more easily. It's really a circle of illusion which works, because it is economically possible for it to work, and because no one can really dispel that illusion (of knowing what "better language" means, for example) yet.

So, to answer your question - what makes languages successful or not? Please do read other answers and pay attention to all the technical details, they are important - but in the end I believe, at least for the last 40 years and some more to come, the answer is really simple: people. It's people, which are social creatures, which have emotions, which are susceptible to manipulation, which are rebellious, which are compliant, which are used to things, which are tired of things, which have wants and fears beyond and above technical matters - it's just people who make languages successful or not. It's almost purely a social issue. Think for a moment - what does it even mean for a language to be successful? Doesn't it mean to be popular with people?