From a more anecdotal and hand wavy point of view I it was also a marketing failure. Don't remember anyone thinking "Dell is cool, I really gotta have a dell PC or laptop". The people I know that got Dell was either because it was the cheapest or because their employee or university got a deal on Dell. Maybe I am just hanging out with the snobby Apple people and freedom loving Ubuntu people.

Building custom machines is cool but do most people know what custom components they want. Does my grandma know how much ram she needs?

Then also there is the whole shrinkage of the desktop market. I feel not many people are rushing out to buy desktops. So if that was significant part of their business, there is less of that.