The NEC is the one exception to the rule that the US is too big and sparsely populated to support dense intercity rail. (For comparison, the distance between NYC and Chicago is longer than the distance between Copenhagen and Zürich, which is to say, it's longer than the longest axis of Germany.)
The reason why we have such shitty HSR on the NEC boils down to two reasons: one, the corridor is too congested with commuter rail (particularly in Connecticut), and two, the route was rebuilt to "modern" standards in the 1930s. Rebuilding it again to truly modern 220 mph HSR standards involves needing to find virgin right-of-way in Connecticut, which is neither easy nor cheap.
The reason why we have such shitty HSR on the NEC boils down to two reasons: one, the corridor is too congested with commuter rail (particularly in Connecticut), and two, the route was rebuilt to "modern" standards in the 1930s. Rebuilding it again to truly modern 220 mph HSR standards involves needing to find virgin right-of-way in Connecticut, which is neither easy nor cheap.