Question

What was the most important factor allowing immigrants to assimilate into american culture ?

Answer 1

First off, American culture is not a real thing, what you would call "american culture", is just a diffusion of may other cultures. 

However,
Some immigrants, and their children in particular, are inspired by the possibility for innovative expression in American arts, culture, and pastimes. which is a key factor of why they do it. Hope this help :)