One of the "rules" of political campaigns is never have a politician play a public part in a sporting event. They will get booed. This is a particular instance of a greater rule: Know your audience.
Sports fans come to a stadium to watch sports. For some it is merely a fun pastime. For others sports is an escape from the quotidian, a literal "field of dreams" where people can indulge their fantasy of sports' greatness through the power of their imagination. For others, cheering for the home team is part and parcel of local loyalty, as anyone who has been in Red Sox Nation during the playoffs knows. No one, repeat no one, comes to a ball game to see, still less hear from, a politician.