YouTube Gaming has published a list (excerpted below) of the top 10 "trending" games on YouTube last month, and while the lion's share of the games are brand-new games, the #2 title, Mortal Kombat X, is over a year old.
That lends further credence to the theory that revitalizing an old game with fresh content (via DLC or re-releases) is a surefire way to drum up fresh interest in the game.
YouTube compiles these monthly charts based not on which games are most popular (otherwise Minecraft would probably always be #1) but rather on which games have the "fastest growth" in terms of both watch time and amount of YouTubers creating videos featuring the game.
That typically means games which launch big during the month appear on the chart, but not always -- last month, for example, YouTube names Ubisoft's big March release The Division as the #1 trending game, but Mortal Kombat X came in at #2 because of developer NetherRealm's decision to release the game's Kombat Pack 2 DLC that month.
Incidentally, NetherRealm and publisher Warner Brothers Interactive also chose to re-release a version of the game with all DLC packed in (including Kombat Pack 2) as Mortal Kombat XL last month, and that probably helped drum up a fresh wave of interest in the game among YouTube viewers and creators.