There is no point in trying to question their decision. Asking doesn't hurt though and you won't get any answer if you don't ask. Try to be positive and self-critical and analyse, where you could have done better. That's the most productive thing you can do for yourself.
Assessments and/or sim-screenings are not necessarily fair or objective. It's the employer's right to select the right person whom they want to give the job to - and that decision is being made by human beings (who have their deficiencies, as we all know).
I've been to the NetJets assessment and sim-evaluation last year. They were very tough but reasonable and fair, I think (I passed, but I'm not working there today). One could actually question whether it really is necessary to present such a demanding English-Test who was perceived to be difficult to pass even by native Englishmen (which I am not) - but that is entirely their choice.
Going to assessments and sim-screenings should only be done when you really intend to work there, but it's actually a good exercise and the next time you'll feel much more relaxed.
Always look at the bright side of life...