Complex sentence: Additionally, the Sullivan family had lost millions promoting the Jackson Victory Tour in 1984, and the revenue from the Patriots was not nearly enough to service the debt--a factor that forced the Sullivans to sell the stadium to Robert Kraft in 1988.
Split into simpler sentences: Additionally, the Sullivan family had lost millions promoting the Jackson Victory Tour in 1984. With most of their money tied up in the team, they were forced to sell the stadium to Robert Kraft in 1988.