What is the most common reason why companies lose regular customers?

If I had to guess I'd say, "complacency" or maybe "conflicting cultures."  That's two reasons, I know.

I've only ever seen a few corporate, business-to-business relationships end, it's embarassing to have to look colleagues from the other departing company in the eye as things wind down.

For retail customers it's different, in that the customers are more numerous, so it's harder to tell why they leave, as they may or may not tell you.  Business customers will usually say, in writing, why things are coming to an end.