What exactly makes these languages "advanced" (as opposed to "complicated" or "difficult")? Or does "advanced" here mean "harder" as in "advanced math"?
I agree with you. But to play devils advocate, if its so large and deep, why do I only hear about Haskell and Scala when language-geeks talk about "advanced" programming languages? Looking at the wikipedia link you provide, there's a list of bullet points by decade: 7 things worthy of a bullet in the 60's, 7 in the 70's, 4 in the 80's, 2 in the 90's and 0 in the 00's and 10's. Has nothing interesting happened lately? Is Haskell as good as it gets?
It's debatable just how good Haskell and similar languages are. Like any language, it has its ardent fans and bitter detractors.
As for development of "advanced programming languages" in general, if you are interested in what's out there, I strongly recommend you browse the archives of the "Lambda the Ultimate" weblog:
http://lambda-the-ultimate.org/
There are a lot of very knowledgeable language designers on there, and news of advanced language features and new languages filters through there all the time. The blog is not as active as it once was, but the archives are still very much worth studying.
Another great resource is the old c2 wiki. Here are some relevant starting points:
http://c2.com/cgi/wiki?search=haskell
http://c2.com/cgi/wiki?search=lisp
http://c2.com/cgi/wiki?search=scheme
Finally, just hang around HN for a while, and you'll see plenty of posts discussing new "advanced" languages. The HN archives are also worth browsing through.
Seriously, though, it takes time. Lisp had higher-order functions and garbage collection long before they became mainstream. The type systems used in OCaml and Haskell were discovered in the 70s. Maybe in 10 years we'll see what of the 90's ideas become really valuable (or crappy but mainstream...).