There are still various ‘plain old soundcards’ out there, and some capable of superb quality as good as any audio interface can get. However, I suspect the move to Firewire/USB is largely because:
a) You don’t have to open up your PC to install them.
b) You can use them with a laptop as well as desktop computer, making them more versatile.
Indeed, with most audio interfaces today offering good quality sound, the connectivity does tend to become the most important factor for many musicians.
As for sound quality, there really isn’t anything between them - the low-jitter digital clocking and analogue circuitry can now produce similarly good results whether the interface is PCI/PCIe/USB 1.1/USB 2.0/Firewire 400/Firewire 800...
However, I still maintain that a well-designed PCI/PCIe soundcard can provide lower latency and potentially more simultaneous audio tracks than either Firewire 400 or USB 2.0, and avoid any of the intermittent connection problems that seem to plague a few Firewire interface in particular