Plenty of PC's come with Firewire ports and they can be easily & cheapily added if they don't (in desktops). It is a standard.
http://en.wikipedia.org/wiki/IEEE_1394_interface
It's as much a standard as serial ports.
The fact is: in today's computing world, most if not all your peripherals are right on the motherboard. That is: Video, usb, wifi even, ethernet, ps2 (kb + mouse).
Notice how motherboards don't have parallel ports anymore? They're obsolete. Not that FW is obsolete, but a typical mb today will be all of the above and sound typically.
a more high end motherboard will have 5.1 sound and firewire, but when people shop for computers, you don't really have that choice.
and you definitely have even LESS choice in laptops.
you can't find a laptop without USB (laptop that was made in the past 5 years anyway). but it's really hard to find one with FW
the cold hard fact is: FW is nearly impossible to find outside of the mac world, or the higher end PC world, and your typical computer buyer won't be looking for the high end computer.
You don't want to have a crutch next time you go buy a computer.
"Oh my god this laptop is perfect! Except for the lack of firewire"
or
"man this motherboard has everything I need"
I've been doing this a long time, and you want to avoid anything that will become a crutch in future upgrades.
I can find you hundreds of laptops for under 300 dollars (albeit most will be netbooks but the point stands), yet I will be hard pressed to say a single one of those has FW