There are, I believe, two factors at work. Well, more than two but I will simplify them. The first comes under the collection of "domestication"; things that change biology. For example, steelhead in the wild spawn and rear in riffles in areas where each fish defends a territory and they feed on benthic foods (surface too, but mostly drift). In a hatchery they are reared in runs at best or big pools at high densities feeding on surv=face foods. Same with salmon, but some salmon (like coho) live in pools in high density situations so the hatchery is not a huge change.

The second factor, the one I consider most critical to steelhead (and ignored by the researchers as I have asked) is the water they are incubated and reared in. Cold-blooded animals have temperature-specific enzymes to facilitate life's biochemical processes and these enzymes are genetically based. A fish has the genes for warm or cold or cool water. Most steelhead programs incubate and rear on water significantly warmer than ambient. They are genetically selected for being able to function in warmer water. released into wild and they do much poorer because they don't have the necessary enzymes. This is why, I believe, that hatchery salmon generally perform better when spawning in the wild than hatchery steelhead; salmon are generally incubated and reared on surface water.

A few years ago they tried some creative steelhead hatchery practices and they got really good survival in the wild; at least significantly better than the standard. They were incubated and reared on local surface waters, but the researchers did not think that water had anything to do with it.