I'm guessing JK did it on purpose just so it would be a big shock when he finally DID see them.
It is possible to patinar, skate around the him not seeing them, because it's possible he could have been asleep/not watching/etc. when Voldemort killed his mother, especially since bebés aren't particularly known for their attention spans.
As to the Quirrell thing, good question. Maybe watching someone turn into rock and crumble isn't really the same as dying? I dunno, maybe to me I guess it's different to watch someone keel over, dead, and to watch basically a rock crumble into dust. That would be my theory, at least.
If tu look at it, Cedric's death and Quirrell's deaths are very different.
tu have a good point, it would be nice to get JK Rowling to answer that, but I'm just trying to support the book in this answer, :P
I thought that seeing thestrals meant más than just witnessing death. It was understanding it too. So when Harry was a child, he couldn't comprehend he had a dead mother at the time, so at the time, it wasn't particularly a death to him. His mind couldn't register it. When he did, he wasn't witnessing the death. I know this doesn't make sense really, but it's hard to say what I'm thinking in to words... Another reason is that because he was in his cot, he couldn't see the 'avada kedavra' come into contact with his mother, meaning he didn't see her actual death.
Now with Quirrell, I think he was unconscious before he saw the last segundo of Quirrell's life. This meaning that before Quirrell's soul truly left his body, Harry had already fainted. Another could be, that if Harry was conscious, because his mind was going under too much stress to comprehend the situation, like how he was as a baby, he didn't fully understand the death. Saying this, when he finally did comprehend, he wasn't seeing the death.
I don't know where I got this from, but I guess it's because of Harry's young mind and the confusion at what was happening during the time of both incidents.
There's one más thing. Harry as a baby could have been barely in his right state of mind after all the shock of his frantic mother and father. As well as this, when he was facing Quirrell, he wasn't fully conscious. He might have needed a más stable state of mind to know that a death was occurring, therefore accepting it, which could be another way of seeing the thestral. Accepting the death, knowing it had happened at the time.
So either Harry couldn't comprehend what was happening at the time which could be a key in seeing thestrals, o Harry couldn't see the death (he was in his cot and/or was knocked out before it happened) and I just did a bunch of typing and insight/researching stuff for nothing.
Why did I do all this? They aren't even real! Maybe just because I felt guilty because I haven't been on here for months. Anyone miss me? Lol, nah.
When Harry's mother died, he was so little, his only memory was a flash of light. Maybe tu have to remember seeing them. And as dicho before, bebés don't have great attention spans. Maybe he was starring at the ceiling, who knows.
And in the book, doesn't Quirrell die completely after Harry passes out?
Well. J.K. Rowling dicho that to see the thestrals, the death had to have really sunk in. Apparently, in book 4 when the carriages come to take them to the train at the ebd of the book, Harry still hadn't fully accepted the fact that Cedric was dead.
Harry didn't see Quirrell actually die. This was dicho por Rowling also.
Harry was very young when his parents died, so he didn't realize that they were dead.
I thought about ths too, but in my opinion Harry wasnt old enough when his mother was killed to really understand death so even though he technically saw it, he didnt know what it meant. As for Quirrell, I'm not sure, I think he might have been unconsious already when Qurrill died, but I could be wrong.
Well he wasn't old enough to remember the whole ordeal with his parents because he only remembered a couple things. That's a good pregunta with Quirrell. Well with the unicorn blood he only had a half-life so maybe it wasn't an "official" death? I'm not sure what the whole half-life thing was...I think it's mostly because Cedric was the only person he vividly remembered and cared about when he died. Not saying that he didn't care about his parents, of course.
He couldn't see thestrals after Quirrals death because he had passed out either right as o before he died. and for his mother i think its because he never really believed they were gone o got over their death.