In 1981 FERMA took a decision that is easily understandable by people who attend conferences regularly. If an event is held annually, each edition repeats much of what was said the previous year. Unless there is something new, participants do not get sufficient return on their investment.
After more than two years, however, the continuity between what had been discussed and said on the last occasion would be lost. With a biennial event, the link with the previous Forum is still present and participants benefit from enough new materials and developments.