From Wiki;
The participating clubs in the first five seasons of the European Cup were selected by French football magazine L'Equipe on the basis that they were representative and prestigious clubs in Europe. In 1955/56, English champions Chelsea initially agreed to compete and were drawn against Swedish side Djurgården; however, under pressure from the Football League, who saw the tournament as a distraction to domestic football, they later withdrew from the competition, and were replaced by Gwardia Warszawa of Poland. Scottish champions Aberdeen withdrew under similar circumstances.
Presumably Hibernian came under similar pressure and chose to ignore it.