30th conference final (or equivalent) for a Boston sports team this century, by the way. That's off 86 seasons between them (and despite 2 years of nothing to start it off, so 30/78 since the start of 2002).
Boston teams make the conference finals 34.8% of the time. Since the start of 2002, it's 38.5% With an average of ~30 sports teams in each league over that timespan and four teams making it that far, the expected figure would be 13.33%