Why DELL lost
There are simple truths. Many great leaders led their company by listening to their customer’s viewpoint and giving them an adequate solution in terms of quality and price (the original Dell model). Then, somehow, Dell lost the contact with the customers, shifting their focus to their own point of view, based on the fact that they were about the biggest computer company, mislead by marketers telling that customers would now (have to) listen to them.
Then came the fall. They started bullying customers, putting aggressive marketing plans in place, deceiving selling techniques, breaking their promises, devaluating models by proprietary standards, and so on. They even managed to massacre the golden consumer brand Dimension (for years THE reference in terms of ‘best buy’, what would you do with a marketer changing the name of Coca-Cola into something else?).
These are probably much more the underlying reasons of their fall than cheap PC competition, because their hardware is still great and customers are willing to pay for quality (look at HP, they’re not cheaper but their customer relation is much more reliable and honest), but consumers don't like companies that don't listen to them...