Regarding citizenship: immigrant must live here at least three years before applying:

An immigrant can become an American citizen by naturalization by living in the United States for a specified period, usually five years (three years if married to a citizen) and passing a naturalization examination. However, there is no requirement that an immigrant become a citizen and s/he is free to live in the United States as long as s/he wishes regardless of his citizenship, so long as s/he abides by the laws of the land, which are applicable to citizens and aliens alike.

Also, according to one website I found, a government one, as of last summer, it costs almost $700 to apply for citizenship and that is non-refundable if you are denied. They do have some provisions to waive some fees if you jump through enough hoops. Those costs do not include any classes to learn about our country, as near as I could tell.

The other reasons I can see for someone not jumping right up to become a citizen would have to do with their circumstances, i.e. was the marriage going to last, do they have hopes of going home, again, possibly having come from a country at war, will they be able to find work and make a go of it, etc. etc.