I think it's just that PC's were out of the gate first and got a good foothold before Apple came along.
Itís the opposite actually, Apple got there foot hold in first with the Mac, PCs got going later. (They had been out a number of years before they really got going thanks to Microsoft)
Apple was a closed system so they had full control and could charge as much as they could get away with, PCs allowed anyone in, thus a boatload of competition meant low prices and masses of software and hardware. Because of this competition PCs virtually saw off Apple, and if they had not been successful with the iPod they would probably now no longer exist.
Apple has now re-built the name for itself and once more people are happy to fork out a boatload of dosh to buy into the Apple brand. (Even though technically in a lot of areas they fall well short of their cheaper completion)
Things however are a changing so it will be interesting to see how they get on in the future. (They havenít held the No.1 phone slot for ages)