I think hotrod nailed it for the cheakamus...access. Like he said not to detract from the squamish, but the cheak is much more accessible, less driving, less hiking...etc...
They are all excellent rivers...I fish the mamquam a lot for steelhead, and enjoy the river immensely, but I haven't really hit too many dollies in there, always the cheak or the squish...
Those rivers rock! No retention I think contributes a lot to that...
rib