I believe it's most likely because they are Canadian. Yes yes, there are a few huge Canadian bands, but if they just don't get that lucky break in the US, then it's just another loss.
I could name many that just never made it huge (though obviously some more than others).
I Mother Earth
Our Lady Peace
Kim Mitchell
Harem Scarem
Honeymoon Suite
Billy Talent
The Headstones

Sure they had careers, but it amazes me that some of these guys didn't become world powers...