There are a number of reasons. The most common is a criminal record. Another problem is an unwillingness to forgo prior citizenship (when dual citizenship is not an option).
Someone can be barred because of criminal actions such as any felony, and sometimes even severe misdemeanors. Also, one can be barred if there are lies on any of the applications for naturalization, including work history
A person may be barred from naturalization for staying too long outside the U. S. A while holding residency or joining a banned organization or becoming a prostitute.
A person can be barred from naturalization if it was found later that they lied on their paperwork or had committed some crime in their former country.
