They were adapted for the American palette and adapted to what was available in terms of produce etc. Many Italian-American dishes developed because the original Italian immigrants were very poor, so there was a lot that they couldn't afford and certain vegetables were not available at all or not available during the winter months. The women often worked actual jobs, so they didn't always have time to make the more time consuming Italian fare. There is also the fact that the preponderance of the Italian immigrants came from southern Italy, so there isn't the same variety of Italian dishes in America that you find in Italy. The idea of eating everything with a sauce is very southern Italian - it was a way to stretch meat in particular. Most of the Italian dishes in America originate in Lazio, Campania and Calabria, and Sicily.