Because it's just not very good anymore. Their coffee is acceptable again, but their donuts are mediocre, and most of their food is just okay. I still like the breakfast sandwiches, though. And Timmies really tried to make their identity about how Canadian they are and that is not true at all anymore, so Canadians feel a little betrayed too. (This is not about their employees, but rather the fact that it they are now owned by an foreign private equity firm.)
The TFW thing is quite bad IMO, but specifically because they are exploiting those workers to an even greater degree by holding continued employment (and thus presence in Canada) over their heads.