I guess it depends if anybody has the time/resources/enthusiasm/knowledge/patience to recompile all the Arch packages to target armhf instead of armel. Have a read of the Raspbian history http://www.raspberrypi.org/phpBB3/viewt ... =66&t=4256
to see what an incredible effort it takes
EDIT: "Standard Debian" supports armel (ARMv4 and softfloat) and armhf (ARMv7 and hardfloat). RaspberryPi has a ARMv6 CPU so Debian Squeeze for RPi (and the earlier wheezy beta) was using armel (an ARMv6 CPU can run code for ARMv4, v5, v6 but not v7). Raspbian recompiled every one of the tens of thousands of packages to target ARMv6 and hardfloat.
From a quick look at the http://archlinuxarm.org/
website it looks like "Standard Arch" supports armv5 (presumably softfloat?) and armv7 (presumably hardfloat?). So Raspi-Arch is more optimised than Debian squeeze was (ARMv5 vs. ARMv4) but not as optimised as Debian Raspbian is.
And similarly the reason that Ubuntu is unavailable for the RPi is that it only targets ARMv7.
EDIT2: Forgot to search http://www.raspberrypi.org/phpBB3/viewt ... 53&t=10901
http://raspberrypi.stackexchange.com/qu ... at-support