Immigration law allows US citizens and Lawful Permanent Residents to petition for permanent resident status (a greencard) for their spouse.  Family-based immigration is a two-step process.  First the US citizen/LPR petitioner must request an immigrant visa for the spouse by proving that the marriage was entered into in good faith, not for the primary purpose of gaining an immigration benefit.  Second the immigrant spouse must prove eligibility for admission to the U.S. as an LPR.  For the spouse of a US citizen there is always a visa available and the case can be completed in the amount of time it takes for processing.  For spouses of LPRs there is a limited number of visas available.  This means spouses in this category must wait for their turn.  The wait times vary between 6 months to 2 years or longer) depending on demand for this type of visa.