Ruby isn't mentioned because it never made significant waves in the educational community. Remember, the article is just about educational language choices, not fashionable languages in general.
The bulk of the article is discussion of Python's "weaknesses":
1) Creating non-trivial data structures is onerous.
2) Limited support for testing.
3) Lack of static types.
4) Difficult transition to other languages because the syntax is quite different.
Show me some real-world examples of a data structure or test setup I can't do better in Python. Python's doctest is an excellent methodology for teaching Test-Driven Design, or even just teaching basic Python (see
pykata.org).
I think the best way to understand these criticisms is to take a look at the language Pyret (pyret.org), a language that is being developed by Shriram Krishnamurthi (one of the educators quoted in the article) as an answer to the problems he sees with Python as an educational language.
Shiriam Krishnamurti & Co. have a long history of being bitter about Python's success and vocal about it's perceived shortcomings.
I suppose it makes sense that the ACM might be getting some input from that corner, which has also given us PLT Scheme and Racket before Pyret.
'Nature" also had an article on Python recently and pointed out that one of Python's chief advantages is the ecosystem to which it gives access, as a controller language.
There's no reason to avoid "teaching languages" such as Racket, Squeak (Process?), Pyret and so on, but I do note they tend to have a shorter half life than languages used widely in industry and they do not open as many doors to 3rd party assets.
I wouldn't give up on on Ruby making greater inroads in education, just maybe not among English-speakers as much.
Kirby