As with almost anything NBA related, the CBA FAQ is a wonderful resource:
Teams are restricted from trading away future first round draft picks in consecutive years. This is known as the "Ted Stepien Rule." Stepien owned the Cavs from 1980-83, and made a series of bad trades (such as the 1982 trade mentioned above) that cost the Cavs several years' first round picks. As a result of Stepien's ineptitude, teams are now prevented from making trades which might leave them without a first round pick in consecutive future years.
(gory details follow if you want to know about how it works in edge cases). As to why, the idea here is that even if a team is rubbish, it will get a high first round draft pick and be able to get better again - unless the team keeps trading away its first round draft pick for mediocre players, in which case it will keep on being rubbish. The NBA likes balance, and the Stepien Rule is a way of ensuring that incompetent management can't completely screw a team over.