It all started with Elben Shira's The End of Dynamic Languages, which sparked Have Static Languages Won? (via) from Maxime Chevalier-Boisvert. In that I read the following:

Like [Elben Shira], I've noticed that despite the fact that there have been an enormous number of new programming languages coming out recently, the overwhelming majority of them are statically typed. [...]

I'm an outsider bystander on all of this, but it strikes me that one possible contributing reason for lots of people creating new statically typed languages is that there is a significant body of academic research that has not yet made it into a popular, mainline programming language. Here I'm thinking primarily of sophisticated type systems and type inference. As long as this situation exists, people tempted to create languages have a clear void that their new modern statically typed language might possibly fill (at least in theory).

(And then they can mingle in some degree of immutability and this and that from other academic research. There's a lot of academic work on statically typed languages that hasn't gotten into popular languages yet. There's also a bunch of people who are grumpy about this lack of popularity, which is another crucial ingredient for creating new languages; see, for example, all of the people who are unhappy at Go for having such a simple and 'primitive' type system in the face of much more powerful ones being out there.)

While I'm not fully in touch with the dynamic language world either, my impression is that there is no similar reserve of well-established academic research on them that has yet to be reflected in popular dynamic languages. Without that, if you're going to create a new dynamic language today it's not clear how you're going to make it significantly different from the existing popular dynamic languages. And if you can't make it very different from the existing good options, what's the point? Certainly you're not likely to get much traction with a slightly different version of Python, JavaScript, or the like.

(Arguably you're not very likely to get significant traction for an advanced statically typed language if so many other ones before you have not been hits, but that's somewhat different in that hope springs eternal. It's the same impulse that keeps people writing new Lisp like languages that they want to be popular.)

PS: I could be totally wrong on this in that maybe there's a pile of good academic research on dynamic languages that's begging to be implemented and made popular. I'd actually like that; it'd mean we have the prospect of significantly better and nicer dynamic languages.