The reason most scientists dismiss the theory of intelligent design primarily because it has not and cannot be tested using the scientific method. The scientific method requires that a theory generally be tested in a controlled environment and controlling for outside variables. Intelligent design is based on observation, but no testing.
On the contrary, there is so much support for evolution that scientists no longer question whether or not it exists, only how exactly it functions. There has been no credible evidence to support intelligent design, therefore sicentists do not support intelligent design as a competing theory to evolution.