Aaaaargh!
Under the old system...
12 pence to the shilling
20 shillings to the pound
2 shillings = 1 florin
2 shillings and 6 pence = a half-crown
(there was no "crown" note or coin)
21 shillings = 1 guinea
(there was no guinea note or coin)
and of course there were farthings, ha'pennies,
tuppence, thruppence, fourpence and sixpence.
A pound used to be an enormous amount of money, so the common unit of commerce was the shilling. With 12 pence to the shilling, it was possible to split up the shilling evenly into groups of 2,3,4,qnd 6 pence.
It's the same reason things are sold in "dozens" to this day. The package can be broken evenly in so many ways. Actually, it makes more sense than the "order of magnitude" idea behind the metric system.