Because it was an arbitary limit set during the days of tiny aircrafts with little weight capacity? Fill a suitcase with some clothes and a few books, and it's already almost 20kg. (Many sturdy suitcases already weigh something like 2-3kg, I think)
Given the weight capacities of the latest airliners, 23kg per passengers is a pitiful amount. The airlines fill the remaining cargo capacity with their cargo operations which earns them a substantial amount of extra revenue. Not to mention the excess baggage charge, which is set at incredible levels (£30+ per kilogram to Asia, for example - 10kg over the limit and that'd be £300+, thank you very much).
I personally think that the USA/Canada allowances (32kg x 2) is much more reflective of the current market and airliner capacities.