Based upon their own projections, Twitch can't maintain this rate of growth.
Sources with Variety report that YouTube is nearing a deal to buy Twitch, the popular game streaming startup, for $1 billion. The deal is said to be an all-cash offer and will close "imminently," according to Variety; The Wall Street Journal, however, has followed up with a report claiming that discussions are "early" and that "a deal isn't imminent." The move, if it succeeds, would effectively put one of the web's most highly trafficked sites firmly in Google's hands.
Presumably, this is why they are entertaining the idea of being bought out.