year-round visibility...
As seen from northern mid-lattitudes where modern astronomy originated.
Don't know for sure about all of them but Canopus is a southern hemisphere star, Sirius is VERY seasonal. I guess that Rigel and Arcturus are also seasonal.
You may also have some colour effects. Arcturus is definitely a red giant and I think Sirius is a Blue giant, so you need a bright star with neutral colour for a good index.