It’s a pretty simple explanation.

Bay Area does not have much industry outside of tech. New York City is a hub for banking, investing, insurance, government, healthcare, consulting, real estate, fashion, law and more. All of these are now increasingly hiring tech workers directly and also retraining and transitioning their existing workforce to be more technical.

So just by numbers alone Bay Area will never maintain a monopoly on tech jobs, especially considering the NYC employment market is several times the size. However if you just filter to the tech sector the numbers will be very different.