Once the grenade exploded, the contents (i.e. the white phosphorus) would scatter and ignite as soon as they touched the air. This made the grenade extremely dangerous — hence its usefulness in combat.
When the war had ended, many of the grenades had become dangerous, due to the corroding of the tin plating. In 1948 the grenade was determined to be obsolete and all were destroyed to minimize the danger they could have caused.
However, these were produced and used in Canada until the 1950s, for the quality and manufacturing of them was better than found in Britain.