They are using the better-working brand for re-entering these markets after the first attempt failed years ago... seems logical to me? The name JustinTV IMHO has a lot of negative baggage attached because it wasn't very successful for so long.
EDIT: Downvoted, maybe I wasn't clear enough. Context is very important within a brand. If you start offering all types of videos on Twitch, the landing page isn't just going to show gaming videos any longer.