> It should be relatively obvious at this point that the outright merits of a language have little or nothing to do with its popularity, at least when compared to things like the corporate backing and will behind a language (see: Objective-C / Swift), and other factors such as positioning and luck (e.g. Javascript). So, you cannot start from "this language is _clearly_ better, so it will certainly become more popular".
Well... it's complicated. I would say that for a new language to succeed now, it has to be clearly better at something - some niche, or some programming paradigm, or some such. It also has to meet the minimum bar of having libraries that cover much of the normal stuff that we expect libraries to cover. (That's where corporate backing comes in - it pays for building all the libraries that people have come to expect.)
But if the language isn't clearly better at something, then it's clearly worse, because I can hire people for a mainstream language, or I can have trouble hiring people for the offbeat language. If the offbeat language isn't enough better to make that a worthwhile trade-off, then why not use the mainstream one?