I suppose I would reply with the question "relative to what?" There is no inherent reason why decimalization is superior to the old system, beyond perhaps teaching foreigners your money and making for easy long addition. Twelve pence to a shilling, 20 shillings to a pound, was a system that had been in use for quite some time, with a 12:20:1 ratio for money being traceable all the way back to Charlemagne over a thousand years ago, well before modern England existed. Simply put, it worked and was familiar, so there wasn't much impetus to change.

Consider, for example, the similar question of why Americans still use imperial measurement instead of metric (decimal!) measurement. Twelve inches to a foot, three feet to a yard, 1760 yards to a mile is the system Americans are used to, and decades of on and off attempts haven't changed that. Same with 8:2:2:4 for fluid measures (ounces:cups:pints:quarts:gallons). The same forces were at play with the GBP, and there were certainly plenty of folks when the change occurred who had more trouble with the new system than the old. When 240d became 100p, that was an odd bit of math to do for a generation that had grown up and grown old figuring prices in the tried and true older way!