This whole move is about corporate governance. The US makes it really easy to start or manage corporations and the courts are (mostly) streamlined and predictable, especially the chancery courts in Delaware. Cayman Islands adopted much of Delaware's legal approach to corporations in 2016 to make the island more business friendly rather than just a tax haven, and they've got a foot in the Latin American market. Singap…
VCs are not going to know that when evaluating a company. YC as the incubator and the first check in has an obligation to vet the situation for future investors. The easiest way for them to do that at scale is to ensure they are experts in a very small number of jurisdictions that are predictable.
Honestly, it makes sense.