Why did immigrant people speak English at home to their children in the 18th & 19th centuries?End of 18th, beginning of 19th.
Well, at the beginning of the 18th century (1700) there were very few people in what is now the United States, and they were concentrated on the eastern seaboard between the Atlantic Ocean and the Appalachian Mountains. Most of them were English, Scottish or Irish, and so they already spoke the language.
At the end of the 19th century (1875 - 1900) the immigration and population of the United States looked very different than it had 200 years earlier. A massive influx of southern and eastern Europeans came into the northern cities. German, Polish, Russian, Italian, Hungarian and Jewish immigrants came in especially high numbers, and brought their native languages with them. Because most of them were employed in factories, working long hours for very low pay, they had no chance, nor enough money or energy, to learn the English language, let alone speak it with their children.
Their children learned English in the schools if they actually attended (many also worked in factories and mines) and ended up translating for their parents when they needed to. It was often the third generation before they lost their heritage language and considered English their native tongue.