I suspect there are three main reasons. Limitations of hardware mean we're increasingly looking at computing in parallel, which is a natural fit for languages

I suspect there are three main reasons. Limitations of hardware mean we're increasingly looking at computing in parallel, which is a natural fit for languages that emphasise immutability. Distributed computation also benefits hugely from immutable values, as it sidesteps much of the problem with cache expiry. We're also seeing a demand for more reliable systems. Taking mutability out of the equation eliminates a significant area of possible bugs, and allows for more sophisticated type-checking. The third reason is simply that it's only recently that we've gotten good functional languages with a comprehensive and mature set of libraries. Haskell has been around for decades, but when I first started learning it seven or eight years ago, the set of libraries on offer was very thin compared to Haskell today. Scala only popped onto the scene 11 years ago, and Clojure only 7 years ago.