Some of the most profitable integrators that I have worked with over the years are the ones that know what they are good at and stick with it. They find one or two profitable market niches and become experts in serving these sectors. They don't try to be everything to everybody.
Along the same lines, they are very strategic in deciding which products to represent and don't take on new products just to meet a spec or to satisfy the needs of a single customer. As a result, they get very good at working with the products that they do choose to represent, and because of their loyalty, often receive preferential treatment from their manufacturers.
For example, I work with one integrator who just does small to medium sized access control systems. He uses one of the second-tier (IMO) brands exclusively, and does tons of work for property managers in the commercial office building environment. Despite requests from his customers and me, he doesn't do video, intercom, or intrusion alarm. His techs just crank out tons and tons of access control projects, month after month and year after year. As a result, they are highly efficient, and deliver excellent quaility work in a unbelieveably short amount of time.
I haven't seen his financials, but by all appearances (and by the prices he charges), this operation has got to be highly profitable.