Chareidi influence is powerful because chareidi teachers often educate MO kids. Chareidim in general also are growing very fast, and the Baal teshuvah movement has exploded in the last 50 years. Minhagim once done by only the pious few are now mainstream Halacha. Cases in point: chalav yisroel, black hats, sheitles, separate seating at events, bugs in lettuce, etc. Overall the trend of frumkeit is heading rightward.
For more on this, read Rav Chaim Solovetchik’s essay on chareidim and the “slide to the right”. It’s called ‘rupture and reconstruction’ and is free online- it’s mamesh a sociological masterpiece. He contrasts mimetic vs. by the book practice, and uses shiurim for kiddush and matzah as his important examples. Personally, if I had to pinpoint a why and wherefore of it all, I’d say it was a reaction to the horror of the holocaust, and the struggle to reestablish life exactly as it was before the war, even if it wasn’t actually done that way in Europe. Strictness and machmir interpretations were adopted to make communities insular and protect them from the outside world, which was a dangerous place to survivors. That’s all I’ve got for you.
And yes, that’s true. A lot of girls I know from MO families are in all-girls bais yaakov type places, rather than mixed.