Why can't the origin be changed on a changeable ticket on SU?
 

Previously, I tried to call SU to change the origin of my air ticket from HKG to CAN, but was told that it isn't possible to change the origin, and I have to cancel, get a refund, and buy it again. However, I have no problem changing the destination of an outgoing flight from HEL to ARN, while keeping the return flight intact (i.e. a simple round-trip became an open jaw)

Why is there such limitation that the origin of the air ticket can't be changed?

You'd have to check the fare rules to know what changes are allowed or not allowed on your ticket.

Rerouting is a change as per SU fare rules, however, I do not know if there are any general provisions in the conditions of carriage that do not allow the change of origin. I was able to change the origin from CAI to IST on one ticket recently, but that was a short haul. Still sounds like a HUACA case to me.