I tend to use a lot of thigh meat as its tastier, but it is more hard work disecting the bone out of it which is why I think breast is used more often.
Also the breast can be cut into larger pieces for tandoors etc.
The reason Chicken Breast is more popular in restaurants is mainly for two reasons.
1. It's not that cheap, but when buying Chicken breast, it is not as cheap as thighs, but the labour involved in turning boned meat into something you want, it is cheaper in the long run to buy the breast.
2. Most people (in a restaurant) especially the lady folk, would prefer not to pick up chicken thighs in a restaurant and start chewing the meat off the bone, so to speak. They would much prefer cut up breast meat that is easy to deal with....
We go through over 200 kilo of Chicken breast and other meat in our kitchen every week, and from a chef's point of view, we make more money serving Chicken breast than any other meat, (portion for portion).