Maybe because CBET's transmitter is closer to Detroit than the others -- near the town of McGregor (it used to be even closer than that -- at its studios overlooking the Detroit River). That and its signal strength (325kw) meant it was subject to blackouts of most American programming, especially shows on American networks (though last year's belaguered "The One" was an exception).
In the case of CKCO -- channel 42, though it covers Port Huron nicely, has a fair to poor signal over Detroit itself. (Channel 13 is too far from the border to be received in the US.)
CIII's channel 22 transmitter is located near Stephenson, far enough from Detroit (and uses a directional antenna to keep its signal away from Cleveland), but until the mid-1980s, its transmitter was based in a small town of Cottam. Someone mentioned earlier that before the transmitter was moved, its American programming (generally during prime time) was literally blacked out -- all viewers saw was a black screen.
As for CHWI, its channel 16 signal in Wheatley is far enough from Detroit, and its channel 60 signal in Windsor is too weak to cover detroit.