Why we don't see the "SkyRider" seats in action?
There are a lot of disadvantages, even if the possibility of lowering costs and increasing profits is a huge advantage for the airlines. This article discussed several issues from a passenger perspective:


Limited space in general (e.g. to use a laptop)
Face too close to the video screen in front of you (if there is one)
Doesn't fit tall, large, short or young people well (and I guess that older people might not be strong enough to use it)
Very limited space for carry-on luggage (because you lose space both under the seats and in the overhead bins)
Passenger resistance to the basic idea


The author's conclusion was that the seat isn't actually uncomfortable, just cramped, but he was very sceptical that passengers would be willing to use it.

Another article added some concerns from the airlines' point of view:


Could make cabin evacuations more difficult (and certification costs in general would be high for a completely new seating concept)
Difficult to market successfully
The manufacturer intended the seats to be used in a new budget fare class, not to replace current economy class seats (this adds more complexity to everything: aircraft layout and maintenance, marketing, booking systems etc.)


Both articles (and others) concluded that there's no fundamental technical or regulatory barrier to introducing them, the main problem would be convincing passengers to accept them. Even Ryanair - which is notorious for cutting costs wherever possible - rejected them although Wikipedia says that both Ryanair and Airbus have proposed an alternative but very similar concept in the past.

If the airlines really wanted to introduce this seating arrangement, I would expect them to keep the idea in the news as much as possible and generally work on convincing people that the seats are comfortable and it's a great way for them to get even cheaper tickets. Since they haven't done that (if they have, I haven't noticed) it seems unlikely that the airlines are actively working on them.