Frankly, there isn't a very strong argument.
Dexter Fowler is quite overrated right now.
But there is an argument and it says that Toronto needs an outfielder, a leadoff hitter, and a guy to help from the left hand side of the plate and Dexter is all of those things.
Also, Bautista is coming off a bad year at 36 and Dexter Fowler is coming off a very good year at 30 years old. The chance that Bautista is toast is real and Dexter is a pretty safe commodity. They'll get a similar AAV and Toronto has limited money so maybe best if they go the safer (younger) route.