Because a U.S. brokerage house has to be a member to the TSX and most do not want to go through the time, expense and compliance procedures unless they have existing investment banking operations in Canada and/or large retail network in the U.S. to make it worth their while.

You could be describing TDAmerica, whom I think (don't know for sure) may have Toronto Dominion Bank in Canada as their parent. Might be worth an inquiry for trading ECU warrants only, and if you have another preferred broker, continue to use them for other trades,