Probably because the first two kids are happy and well adjusted... and the whole family tends towards autism without any of the things that could cause early death--you know, seizures, autoimmune disease, severe allergies, Rett syndrome related nervous system problems--so if you're going to have kids who are reasonably happy and won't die early, I don't see what autism has to do with it. We can always use reinforcements.
I mean, I'm the oldest in my family, and I have two younger siblings--one of whom is also Aspie--and what does that tell you? Mind, my mom didn't know I was autistic until I was 11 years old; but I was definitely a difficult child, with the constant tantrums, insomnia, and utter insistence on total fairness... anyway, the littlest one is 11 and a half years younger than me...
OK, considering the 14 year old Aspie--come on, that's a really easy mistake for any of us to make. If somebody makes a statement that only suggests a question, we may easily decide to err on the side of caution and not "answer" it. Lots of Aspies are shy that way, and anyway, the guy was being filmed--even more reason not to embarrass yourself. Silence is a lot less embarrassing than saying the wrong thing. And not noticing that it suggested a question is not unlikely, either; you might simply assume that, as it usually is, a statement like, "I have a favorite character too," will be followed by the answer to the question without the other person having to ask the question at all. If anything, expecting a question is too artificial. That confusion on top of being filmed, and wanting to be cautious anyway, could easily end up with enough delay while trying to figure out what to say and if to say anything at all... that the impression would be "This boy doesn't know that he's meant to ask a question.
I know that's overanalysis--but I betcha that kid was making the same overanalysis I just did, or something similar. (Or he didn't like the shrink. Hey, it's a possibility.)