Well, I would say that it is a combination of experience equity, professionalism, chemistry and of course price. Some brides act more on emotion than economics, so it is not purely a cost issue. I would recommend creating a portfolio of your work. Shutterfly has great templates for hardbound books. I find this style of portfolio can showcase your work in a professional package.
I know that the reasons I would hire someone is trusting their product or service. Knowing that they will give me just what they say and better. When we hired our wedding planner for our wedding, I had absolutely no doubts that it would be exactly as she said, and we now run our business the same. There is really no way that we would not deliver what we had discussed and more if possible. Hope that helps!