Off the top of my head, US-EAST-1 is:

(1) topologically closer to certain customers than other regions (this applies to all regions for different customers),

(2) consistently in the first set of regions to get new features,

(3) usually in the lowest price tier for features whose pricing varies by region,

(4) where certain global (notionally region agnostic) services are effectively hosted and certain interactions with them in region-specific services need to be done.

#4 is a unique feature of US-East-1, #2-#3 are factors in region selection that can also favor other regions, e.g., for users in the West US, US-West-2 beats US-West-1 on them, and is why some users topologically closer to US-West-1 favor US-West-2.