One reason, a US based ETF gets access to BTC to capital in accounts with limited permissions.
Many US retirement investment accounts are limited in what assets they can hold, so a Canadian or international ETF is not available.
Thinking more adversarially, there are ways an ETF can influence the future of the network. For instance, they could engage in shorting & price controls in a similar manner to what folks allege has been going on with gold for decades. Another way is by promulgating their own fork of BTC and shorting or refusing to hold coins that are not their fork.