I don't think the number of people matter as much as the number of competitors. NYC has 20.3mm people, most of whom either don't care about sports, or have 3-4 major teams they follow so they think "I don't need soccer" to the extent they think of it at all.
Would be sports fans in Portland and KC think "might as well try soccer" for lack of options.
And worse, too many existing soccer fans in NY look down on MLS. So the market is a subset of existing soccer fans who don't share that bias, or folks like me who became disenchanted with some of the major sports and only then said "might as well try soccer."