The Phils were in the middle of a general dismantling of the team that won the 1980 World Series. The off-season following the 1981 strike and playoff loss to the Montreal Expos was very busy... as the club traded Lonnie Smith, Bake McBride, Bob Boone, Keith Moreland, Dickie Noles and Larry Bowa.
Bowa, whose best days were behind him, was in the middle of a contract dispute that angered owner Bill Giles. Rather than negotiate with Bowa, he was sent to the Cubs for shortstop Ivan DeJesus. At the time, the trade seemed like a good deal for both teams. The Cubs needed Bowa's veteran leadership and the younger DeJesus had more range, and a cannon of an arm. Despite his lack of offense, he fit the needs of a contending team like the Phillies.
From the book Almost A Dynasty by William C Kashatus:
Sandberg was a "throw in" in the deal. He had only played 13 games for the Phillies in 1981 and hit an unimpressive .167. Nor did he fit into the team's immediate plans. "The Phillies knew they weren't going to win another championship without a proven shortstop and that shortstop wasn't going to be Larry Bowa because of his bitching with Bill Giles" said Green, downplaying the notion that he fleeced his former employers. "Ivan DeJesus was a proven - and marketable - shortstop. At first the Phillies tried to keep Ryne Sanberg from us. But I insisted on him if was going to make the deal. I think the Phillies knew Ryne was a good athlete. They just had no place for him for two or three years. They were going for a pennant and there was some skepticism he could play shortstop in the majors. Schmidt was at third. I always thought Ryne could play center field, but Maddox was there. The Phillies never really thought of him as a second baseman, and besides, Trillo was already there."
Ryne Sandberg went on to become the finest second baseman of his generation. In Chicago became the face of the Cubs franchise after he won an MVP and nine Gold Gloves, made ten All Star teams and hit 282 HRs. He was elected to the Hall of Fame in 2005.
Ironically, DeJesus did something Sandberg never could... playing in the World Series after the Phillies won the 1983 NL pennant. He was traded the Cardinals in 1985.