My guess (this might've been said already) is that there just isn't a model ready in any of the current builds - there isn't much point in having a selectable brand with no cars in it.
Also, if you notice, Subaru isn't listed in that first image, yet it was in the first public build. For that reason I'm betting barring sudden legal/license conflict, any brands you've seen on any sort of public or promotional material will make it to the full game - less than half of the content car/track-wise has been revealed, so there's a lot to come yet.
I do think it's odd that so little new content has been shown (two-ish new cars and extremely brief and obscure video footage of the unnamed dirt track I believe?), especially for one of the biggest gaming events of the year. However there's still quite a few more shows and events coming up that'd allow for more extensive reveals and coverage. It wouldn't surprise me if Sarthe gets revealed at Le Mans (especially considering the ties to the Bugatti announcement), and if the Copper Box event actually "leaked" it's existence, Pax Prime wouldn't be a terrible place to show off a playable build of Seattle. As for cars, in the past PD has revealed full lists anywhere between a couple weeks to a month before launch, but they could also take a T10 approach and slowly unveil the list piece by piece to steadily build hype up to launch day.