Because it's a separate marketing term.

Instead of the CEO mandating that the API server has to be agent compatible (where who knows what that means), they can just say "our product has an MCP".

On a technical level, who knows what it actually is (is it actually the new stateless version, does it have all the endpoints, is the regular API more feature-rich, do I need those features for my workflow?, etc.). But at a surface-level, the intention is clearer, and lets other gears (like sales and marketing) keep spinning without getting bogged down in technical details.