Do you know why? Bob had a good picture of what can happen to RMC if the available fault current is high enough.EMT is allowed in WA state for service conductors. It is not allowed once those conductors head inside the building.
Per WAC 296-46B-230-43
(8) Electrical metallic tubing must not be installed as the wiring method for service entrance conductors inside a building. Existing electrical metallic tubing, installed prior to October 1984, which is properly grounded and used for service entrance conductors may be permitted to remain if the conduit is installed in a nonaccessible location and is the proper size for the installed conductors.
Do they allow PVC? It doesn't carry fault current, but will melt down if the enclosed conductors get hot enough, I've seen that before, even on load side of service. High impedance faults and 480 volt systems sometimes can burn a lot of things up without tripping overcurrent devices.