The twice-yearly update was introduced in March 1999 following years of dissatisfaction amongst the motor trade with the annual August update, as it tended to mean that few people bought cars in June/July, and also coincided with the traditional holiday shut-down period for car factories, which affected the supply of new cars. The March/September arrangement was aimed at smoothing out demand, with the added hope that more frequent changes would actually make people less likely to hold off for the latest plate (as it wouldn't remain new-looking for as long).
Incidentally, when the original "A" suffix was introduced in 1963, the registration year ran from January to December. This was moved to August in 1967 (with the change from E to F), again due to complaints from that motor trade that the January change led to demand being highest during the Christmas/New Year holiday period. That's why there tend to be fewer E-suffix cars about than others from that era (though it's hard to tell these days).
