If I had to give the shortest answer (and simplest explanation) possible, it would be that none of the small, independent car makers could buy their steel cheap enough. Their size did afford them the economies of scale.
Long version here: http://uniquecarsandparts.com/lost_marques_hudson.htm
I was 6yo when the Hudson marque disappeared and the above article should answer most questions.
Hudson Motors was in trouble long before the 1950s
The 49-54 Hudsons were slick, elegant and fast, but not overwhelmingly so.
The last of the Hudsons from 55-57 picked up the nickname "Hash", because they were built on the Nash floorplan with most of its body work. Nobody was very excited about Nash at the time, much less a rebadged (and somewhat uglier) one.
Mechanically, the Hash wasn't a bad or unreliable car. It just didn't offer much of a reason to choose it over its contemporaries.
In contrast, the last of the real Packards (55-56) were loaded with goodies and solid engineering, very competitive. Didn't save Packard either.
The big success car of 1955 was the DeSoto. Not hard to see why
The competition was producing some elegant cars and other dazzlingly beautiful cars, many of which could be had for the same money for a stodgy, clumsy looking Hash. The classy (and cheap) Ford Fairlane, the Chevy BelAir, the Dodge Lancer, etc. Virgil Exner of Chrysler disrupted American car styling, and put everybody else in catch-up mode.
Additionally, once the stink of death is on a car brand, the public stops buying them. Nobody wants to get stuck with an orphan with no repair shops or replacement parts. I wouldn't buy a new anything from Chrysler these days, they have one foot in the grave, AGAIN!
hope i answered your question from a young kids eyewitness point of view.
