Chasidim today represent about three fifths of American Orthodox Jews. Shocked they’re such a majority of Orthodoxy, are you? It’s easy to miss realizing this statistical fact since Chasidim mostly congregate living in Chasidish neighborhoods and towns. Never been to Kiryas Yoel, like the vast majority of Orthodox Jews haven’t been to it? Well, KY is already half as big as Monsey; and Monsey (which is one of the Big Three areas of American Orthodox Jews [the other two being NYC/Brooklyn and Lakewood]) is itself majority Chasidish, as is Brooklyn. And KJ is constituted of just one chasidus of many, and isn’t even a majority of that one chasidus.
So if you’ve rarely or never visited Williamsburg, Boro Park (which only became vast majority Chasidish over the last 20 or so years; until then it was a mix of Litvish, Chasidish and MO — even Rav Aharon Kotler lived in BP), New Square and Kiryas Yoel, you probably have no idea how demographically huge the Chasidim have become as a majority proportion of Orthodox Jews.
Even Lakewood is now getting a huge influx of Chasidim.
Birth rates matter. And there’s a large statistical difference in that rate between the Chasdim and the Litvish.