My assumption is that stars have significantly larger volume than black holes. And if they spiraled they would would merge before they had the chance to reach high enough velocities to generate gravitational waves detectable by the current LIGO.
Is this the main reason why we know LIGO detected black holes?
Another idea I had was mass. But if black holes can have masses like 30 solar mass then the stars by which they formed would be at least so massive if not more. Right?