When I was planning to apply for TN, I was very hesitant to pack all of my things, buy a plane ticket, go to the airport and have to deal with a random unfriendly customs agent, then have a good possibility of getting declined for arbitrary reasons.
This not only hurts the candidate, but harms American companies so much more. Some technical recruiting process take months to find a good lead for a hire. More specific the criteria, even tougher to find a good candidate. Naturally, American company have to consider beyond American pool of hires and check Canadian candidates as well.
The company easily spend between $2000 to $3000 for TN application, lawyer fee, flight, hotel, other accommodations for interviews per candidate. It's devastating when you try to hire a Canadian after spending so much time and cover all the cost, just to be denied by border officer.