What Stranger said are true, plus in my opinion (rather limited, and not purely based on facts, just pure opinion, so take with tons of salt), not many things needed the speed and bandwidth of PCIe.
1.Most (if not all) boards (enthusiast-based especially) have integrated gigabit Ethernet controller on-board. Other than graphics cards, this is the only thing that I can think of that needed speed and bandwidth more than other things
2.Sound cards, even the most powerful ones, does not need the bandwidth as much as the graphics cards
3.Most Average Joe and Jane don't give much d@mn about the differences, plus most can't pronounce it right
4.There's still a lot, really lot of older boards with PCI slots and no PCIe slots
But still, I personally think the motherboards manufactures should have made better board layout/ design with more PCIe slots while add-ons manufactures produce more PCIe cards
Sorry for kinda bad english, and as I stated before, this is only personal opinion, not 100% facts