Changing digitalWrite will break a lot of code. The fastest form of this
digitalWrite(13, HIGH);
digitalWrite(13, LOW);
results in a 125 ns pulse on a 328.
The pulse is about 30 times longer with digitalWrite in Arduino 018.
The value of Arduino hardware/software is ease of use for the novice. Much of that is due to existing examples, articles, and books. It is not worth breaking these for a few developers.
I suspect many developers wouldn't use the fast version.
For example I wouldn't use it in my library for playing audio on the Adafruit Wave Shield. The hardware SPI is used to read from an SD card so it uses bit-bang SPI in a timer ISR to drive the DAC. It must send 88.4 KB/sec to the DAC for 44.2 ksps Wave files. I just want total control over optimizing this small but critical part of the library so I would not use a new fast digitalWrite.