The cost of incorporation in Delaware is higher than in many states ($50 in Colorado, for example). ChuckMcM's cousin post pretty much covers the reasons. The only thing he left out was the 0% corporate income tax rate. Nevada often comes up for this reason as well.

edit (re grandparent): Delaware is definitely not a "safe haven" for litigation. It's more predictable for the basics, but it isn't going to prevent you from an Eastern Texas District Court summons.