I have a friend from Pittsburgh. From what I gather, the Red Sox and Yankees each play one series in Pittsburgh roughly around once every 5-10 years. So, when you have one of the two most popular franchises coming into town for a very rare series in Pittsburgh, tickets get sold quick, and since its a weekend game, that doesn't help anything. She doesn't get to a lot of games, but when the Yankees were there last year, she made sure to get tickets. I don't personally know anyone traveling to Pittsburgh, but Sox fans have been known to travel, and its not much different in distance than Toronto/Baltimore.
Also, not all tickets went on sale yesterday.