That's a great question.
Again, I do have personal experience in this space, but I'm not aware of any tax limitation on why somebody from the U.S. would not invest in Canada. That being said, there is a preference for American investors to try to invest in the United States, potentially because there are states that have very favourable labour laws and very favourable acquisition laws.
Oftentimes, when an American company or American investor wants to come into Canada, they will advise the Canadian company to set up a U.S. Delaware office. I am aware of that, but I don't believe there is anything fundamental—at least, off the top of my head—that would prevent an American company from directly investing in Canada, other than just an intrinsic fear of the unknown.