Updated: Apr 25, 2020
The CFL is by most standards an inferior league to the NFL from a talent perspective. However, it's still the second-best football league in the world, so if the NFL has the top 0.1% of players, the CFL gets the ones from 0.14% to 0.11%.
Other factors are:
Relatively low pay. You can earn a decent living in the CFL compared to a regular job but by no means do you set yourself up for life.
For American players, Canada can be a strange land. Some love it, but it's too far from home for many to make the move.
For American players, it can be tough to crack the roster because there are limits to the number of non-Canadians who can be on a CFL roster. So the number of opportunities is smaller.
For American players, the rules can take some adjustments too. As great as NFL players are, the game is different enough that not all NFL players would automatically make a CFL roster.