In the past Stubhub generally did not post tickets for Yankees until Jan/Feb before the season begins. This past year they began listing tickets for Yanks in November.
BTW, the only Yankee tickets available on Ticketmaster are for games where the Yanks will be visitors as no home games are listed. This implies that it varies by team rather than all of MLB games.
Look back in November and I suspect you will have better luck.