The main reason for this is that these two groups are in very different situations and therefore have different interests.
When immigrants first come to the US, they often want to take any job they can get. They also typically want to, in some ways, stick together so that they can feel less alienated from their new country. This means that new immigrants end up with the worst jobs and they tend to still act in ways that were more normal for their country of origin.
This can be very different from what longtime residents want. Longtime residents have assimilated into American culture. They typically have come to be culturally much more like Americans than they are like people who have just come from their old home country. They have jobs that are more like the jobs that natives have.
What all of this means is that the two kinds of immigrants will not be very similar to one another. In fact, there can even be tensions as the older immigrants come to feel as if the new immigrants are an embarrassment because of the jobs they have and the ways in which they behave.