On Delaware, because many corporations incorporate there (or incorporate a subsidiary there). It's strange this hasn't been mentioned yet, but the primary reason Delaware is so popular is because it's a domestic & international tax haven [0][1][2]. It's one of the reasons the US is ranked 3rd (of 102) on the Tax Justice Network's "Financial Secrecy Index' (behind Switzerland and Hong Kong).

Interestingly, Delaware should be especially attractive to 'IP heavy' companies, and other companies that have large amounts of 'intangible capital' (e.g. tech companies), as Delaware doesn't tax earnings generated from 'intangible assets'. So if you're a large multi-state / multi-national corporation, you set up a subsidiary (or incorporate your HQ) in Delaware, dump all your intangible assets there, and then charge your related entities fees when they use these intangible assets (e.g. IP & trademark licensing fees).

These fees can then be claimed as deductions in the State / Country the related entity happens to be in (unless the jurisdiction has strong transfer pricing laws that outlaw the more egregious instances of this practice). As with all tax-havens, it's the 'beggar-thy-neighbour' approach to attracting 'capital investment'.

[0] http://www.nytimes.com/2012/07/01/business/how-delaware-thri...

[1] https://www.theguardian.com/us-news/2016/apr/06/panama-paper...

[2] https://en.wikipedia.org/wiki/Corporate_haven#North_America