The reason is that the adaprots that are card specific, are not true pc card slots, they rely on the card behaving as a PCI rather then a cardbus/pcmcia card.

There are some real ones around which when you install them in the computer you get a PCI-PCMCIA bridge arrangement as you have in a laptop, and it gets you hot swapping and working with all cards, but they are hard to spot from all the wireless crap that is floating around, and they will cause all sorts of resource conflits when you use them in a well populated machine. I think ricoh of all people made one some time back, but im not 100% sure if it was the one I saw.