I think you need to be a bit careful in comparing FTA ratings with YouTube numbers.
As I understand it (it's many years now since I last participated in the ratings surveys), FTA ratings show the estimated viewers of the live FTA broadcast in Australia, sampled from the population of households which possess a television set. (I think - please correct my understanding if it's not accurate).
YouTube also shows live A-League broadcasts, although not in Australia (as far as I am aware). Therefore, YouTube figures for live A-League broadcasts are drawn from a population that is not living in Australia. People such as myself. I live in the Philippines and YouTube is my only way to watch live A-League. The broadcasts attract a number of viewers, for example the current match being shown is Brisbane v WSW and just prior to halftime, there were about 6000 people viewing the live stream. Other matches featuring more popular teams than Brisbane will usually draw considerably more live viewers.
The other thing with YouTube is the viewing of replays. Here, instead of live viewers, a figure of the number of times the video has been viewed is given. Whilst I am sure this is probably a meaningful measure of popularity, it doesn't compare like with like - ie live YouTube viewership v FTA ratings for the live broadcasts.
I think what all this goes to show is that continuing to focus on FTA ratings as a meaningful measure of A-League popularity (or interest) is a very flawed concept.