From the link:
One of the major innovations in FP that has resulted in the "explosion" of interest is monads.
In January of 1992, Philip Wadler wrote a paper called The Essence of Functional Programming which introduced monads into functional programming as a way to deal with IO.
The major problem with pure, lazy, functional programming languages was utility in dealing with IO. It's one of member of the "Awkward Squad" in programming, because "laziness and side effects are, from a practical point of view, incompatible. If you want to use a lazy language, it pretty much has to be a purely functional language; if you want to use side effects, you had better use a strict language."
The issue with IO before monads was that maintaining purity was not possible for programs that were actually useful for anything. By IO, we mean anything that deals with changing state, including getting input and output from the user or environment. In pure functional programming, everything is immutable, to allow laziness and purity (free from side effects).
How do monads solve the problem of IO? Well, without discussing monads too much, they basically take the "World" (the runtime environment) as input to the monad, and produce a new "World" as output, and the result: type IO a = World -> (a, World).
FP has therefore entered more and more into the mainstream, because the biggest problem, IO (and others) has been solved. Integration into existing OO languages has also been happening, as you know. LINQ is monads, for example, through and through.
