Can't get a TN for investment banking, it's not a supported occupation (one of my friends had to do this).

Similarly for a few other occupations. And, of course, some want to hedge their bets in case they do want a green card in the future.

It's not dual intent for one thing. I have a TN but my company put in for H-1B for me so that I could switch and then have the option to get sponsored more easily if I decide I want a green card.

TN you can do it, but it's messier.