Because his shtick is that you need to prove that Oswald didn't come down the stairs.
People who accept the Warren Commission conclusions almost universally don't understand burden of proof.
Oh yeah, back to why Oswald went to the movies...
He liked movies. And popcorn. And there was no more work that day.
And he liked staring blankly at tennis shoes, apparently. Not interested in cop cars speeding by with sirens wailing, though. Smith, Wesson, and Lee... Dirty Harvey. That's it, it's all over now. Poor dumb cop.
People who reject every WC conclusion don't understand or accept standard of proof.