As noted, optics are on the restricted/prohibited US State Dept. 'list'.
Most models of ACOG's are able to be ordered through the dealer listed on the Trijicon web site. The supply to Canada is tenuous due to these restrictions as well as the small market size.
Just as most of the Eotech models are available,( from time to time...nothing to do with our importers or dealers !), some, such as the 553, are not.
Just like our Canadian firearm laws, none of this has to make any sense!