B cell maturation antigen (BCMA)-targeted chimeric antigen receptor T cell (CAR-T) therapy has transformed the treatment landscape for relapsed or refractory multiple myeloma (MM), with products such as idecabtagene vicleucel and ciltacabtagene autoleucel achieving high initial response rates, and in selected patient populations, durable treatment-free remission. However, a substantial proportion of patients still experience relapse, including antigen-positive progression, highlighting persistent limitations in long-term disease control across diverse clinical settings. An increasing body of evidence indicates that resistance to CAR-T therapy in MM is driven not only by tumor-intrinsic factors, but also by extrinsic pressures imposed by the bone marrow microenvironment (BMME). This review integrates current understanding of tumor-niche interactions that impair CAR-T persistence, trafficking, and effector function, including immunosuppressive cellular networks, inhibitory cytokine signaling, metabolic constraints, stromal adhesion, antigen modulation, and marrow remodeling. This review further examines emerging therapeutic strategies and next-generation CAR-T platforms.