Or good products, or flexibility, or good price, or good marketing or good sales staff, or because they can deliver 10,000 identical laptops if you order those, or just because they are big.
The last point is interesting: if many use them because they are big, they stay big without much relation with any special factor to explain it: That's like it was 30 years ago: 'You're never fired because you buy blue'. (blue was the color of IBM).
Is it because of good customer service and warranty? sorry for poor english

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic