The main reason is they can’t just go and buy a stamp and away you go.Shipping to Canada or vise versa needs to have a declaration form  stating the contents and most vendors are lazy to fill one out.The other reason is they don’t know what is and isn’t allowed without proper import or export papers and don’t want to take the time to find out.Even a lot of people on ebay,gb,etc are the same but you do find the ” good ones ” that will ship.These are just my findings and experiences and I know a lot of people here in Canada that will say the same thing as well.It really sucks when we have limited access to parts and there’s a ton in the USA but not willing to ship.The vendors would double their sales even though it costs us Canadians more in the end with the exchange rate.