Samca,
They want pilots and assessments cost a lot of money. You say there were 3 TRI/TREs there and the chances are that they all had a good chat afterwards as to how they thought you had done.
There is always more to it than just flying the plane and it is likely that they weren't happy with your communication, CRM, decision making, etc rather than it having anything to do with your physically ability to fly the plane.
They would also expect experience and decision making commensurate with your experience level so if you are a 3000hr B737 Pilot then the reality is that they would expect you to make decisions and fly like a captain. If you were 501hrs on type and 1501hrs TT then they will expect less from you.
One of the other prevalent things that is obvious from your post is that your English could have easily been a factor in all of this as well. I'm not saying it was as I wasn't there but it would have an effect if your spoken English is similar in standard to your written English.
Not getting through when you think that you should have does not automatically make a process unfair I have numerous friends here who were turned down by FZ and who are now at Emirates and Etihad so it may be that they also just didn't think that you would fit the airline.
You have also made numerous posts in the past about whether or not you thought it worthwhile to pay for assessment, whether the airline was far, whether it was for you or not due to family/staff travel/ commuting, etc, etc have a good look at yourself and see whether it was likely that any of that came through in your assessment overall. Did you answer the questions a bit hesitantly and seem unconvinced?
All of these things to an experienced recruiter show up much more easily that you would like to think it is likely that there was more to it than just the sim.
Good luck in wherever you end up but use the opportunity to reflect on how you conducted the entire selection process and take something away from it for when you go to Qatar, Etihad, Emirates etc interviews which I am sure you will aim for if/when you have the requirements.