Why has immune "checkpoint" therapy failed in most clinical trials?

Cancer immunotherapy targeting the PD-1/PD-L1 pathway has demonstrated efficacy across a range of common solid tumors and some hematopoietic malignancies. Despite these groundbreaking successes, the clinical development of other 'checkpoint inhibitors' targeting molecules like TIM-3, TIGIT, ICOS and others, has largely fallen short, often showing minimal clinical benefit even in combination with anti-PD therapy. This article explores three key hypotheses that help explain the disparity in therapeutic success: (1) the absence of tumor- specific immunosuppressive logic in many checkpoint targets, (2) the dominance-but not redundancy-of immune evasion mechanisms within the tumor microenvironment (TME), and (3) the emergence of therapy-induced resistance. This is not intended as a comprehensive review of the literature. Instead, it highlights select evidence to explain past failures and to illuminate a more strategic, biologically informed path forward.