The first reason that comes to mind is the one everybody knows: the PC is almost dead.
But, there is more to it that that.
- Dell never had good products. It had cheap products. Mediocre products. Not having good products made people look for alternatives and as soon as it found them. Dell existed only because of Microsoft.
- Dell did not had a good lineup. They had 3 or 4 line of cheap laptops, one uglier than the other, one useless than the other. Ugly, cheap, hard to work on and more. HP is in the same situation, they have a lot of dumb laptops too.
Dell existed only because the market allowed it to exist. The moment they had to prove themselves they fell on their nose. They had the gorgeous Adamo, a notebook meant to compete with Macbook Air, but they stop producing it after a year. HP are even dumber, everybody was producing 13′ ultrabooks, they thought they ment something and came up with a 14′ one. Did it last 6 months in the stores before they launched the normal 13′ version.
Meanwhile, Lenovo was eating their lunch. And Samsung came up with a couple of beautifully designed models.