Because they decided that there is no point: they showed all that they wanted to show and terms Bethesda and Fallout are by now so strong that they are hype enough by themselves. Todd likes keeping gameplay and story spoilers to minimum and producing videos costs a damn lot, much more than it is worth to them.
To show the quality of the game? You won't see any of that as they will cherry pick and plan it all out to look as perfect as possible.
You are new on the Internet, aren't you? Leaks are common everywhere nowadays. If it is popular, it WILL be leaked.
As far as Beth games go, FO4 is quite light on leaks. OB and SR had many, many more leaks a month before their releases than FO4 does around a week away.
It is my personal belief that TES6/FO5 will be announced at a convention, Todd will show trailers and talk about the game and then someone will ask him "when doest he game come out?"
He will put a hand on his hip, look at the watch on his hand and say: "Midnight. Meaning... in twenty minutes."
And as chaos erupts in the audience and within the staff, he will put his hand on his hips and, with a straight face as if nothing happened, he will moonwalk away.