Code inertia. Huge amounts of existing code written in COBOL = prohibitive costs to switch everything over to another language. Wikipedia says there are over 200 billion lines of COBOL code in use.
Policy inertia. The places where COBOL is really in deep use tend to be government agencies and large businesses, which are notoriously slow to change.
Human inertia. People who make their living writing code and know many languages are less likely to consider it a big deal to learn a new one. People who learned one language because they needed to know it to perform what's otherwise a "business" job may not even think to switch.