it is very expensive to run all the wires to connect things via a parallel bus, that is why drive interfaces have moved to serial busses
trying to map your flash storage into your address space is going to be expensive, and it also isnt very portable from system to system.
it's already hard to get enough slots for everything needed in a server, dedicating one to flash is a hard decision, and low-end systems have even fewer slots.
there is one commercial company making PCI-E based flash drives, their cost per drive is an order of magnitude higher than the companies making SATA based devices, they also haven't been able to get their device drivers upstream into the kernel so users are forced into binary-only drivers in their main kernel. this is significantly worse than the SATA interface version because now mistakes in the driver can clobber the entire system, not just the storage device.
sorry, I don't buy into the 'just connect the raw flash and everything will be good' reasoning.