Yep, CF cards are 16 bit wide parallel, but most of the current advances in bus speed have been in the serial configuration. Examples: 10Gbit Ethernet, PCI Express, 3000 Gbit Infiniband. They just use more lanes to get higher speeds
I thought they increased the serial data speed over the past few years.
Imaging how much faster 16 bit parallel would be with the same clock speed.
Data/address buses in computers are parallel for a reason.