NoSQL databases did not become popular because relational databases failed; relational databases are still alive. They became popular because systems changed.
As applications grew more distributed, data volumes increased, and access patterns diversified, the limits of a single persistence model became more visible. Document databases simplified aggregate storage, key-value stores optimized for latency and scale, column databases handled massive datasets efficiently, and graph databases modeled relationships that relational schemas struggled to express. Over time, these technologies moved from experimentation into critical, production-grade use cases, including highly regulated industries such as finance.