I'm curious as to why flossing targets sockeye more than other fish, and why using spin and glows targets chinook and others.
I wouldnt say flossing targets sockeye more, but rather it targets all fish indiscriminately. If people get more sockeye while flossing, it's probably due to the fact that during the peak of the sockeye run, they are present in far greater numbers than literally everything else combined. Literally millions of fish.
As to why springs will take a bar fished Spin-n-glo, while sockeye ignore such offerings is a good question, and as Mr. Gadsen notes is mostly a mystery, though theories abound. While I dont believe chinook actively feed in rivers it surely must be triggering some sort of feeding/territorial instinct that causes them to bite.
For some reason once in fresh water, sockeye are mostly unwilling biters. People definitively can get them to actively take in clearer waters like the Vedder and Harrison, but in the murky Fraser, not so much, hence the proliferation of flossing. It's the only reliable method of taking sockeye in the Fraser by rod and reel that we know of so far.