For the secondary to be imploded by the hot, radiation-induced plasma surrounding it, it must remain cool for the first microsecond, i.e., it must be encased in a massive radiation (heat) shield.
The initial impetus behind the two-stage weapon was President Truman's 1950 promise to build a 10-megaton hydrogen superbomb as the U.S. response to the 1949 test of the first Soviet fission bomb.