Because given the same chips you can push ECC RAM to speeds beyond what non-ECC RAM can do before corrupting. You can push beyond what the chips are capable of and rely on the error correction to keep your system stable.
ECC in common desktops/workstation/servers will correct all single bit errors and detect all 2 bit errors.
So sure you can run dimms every so slightly faster and fix the occasional single bit flip, but even a single double bit flip and an process or your kernel crashes.
Seems much saner to go for a safe, robust, and reliable system at standard clocks in ECC instead of trying to get slightly more performance which increases the chances or errors, corruptions, crashes, and shorter service life.
