No competent network engineer wants to work in Iran, so government doesn't know how to block v6 properly. End result: just get rid of it entirely!
Why would they want to block IPv6 specifically?
A lot of anti censorship organizations have trouble getting more IPv4 /24 for cost reasons or moving it around to different AS since they would go offline.
With IPv6, you can get IPv6 /40 from ARIN/RIPE no problem. You slice that up into /48 and just start bouncing it all over the place. When one /48 goes down, you move everything to another /48, switch providers if required and continue.
EDIT: They also tend to get multiple blocks as well for when ISP figures out to root /40.