If you only have a hammer, everything looks like a nail.
The concept is thus: individuals who are incomplete in their knowledge or training of solutions propose the same type of solution to every problem they encounter. They opt for the more familiar solution to one that may be more effective yet with which they are unskilled.
In closing, another colleague of mine recently used, perhaps invented, an analogy that provided a new take on the 'sledgehammer to crack a nut' idiom. He considered the introduction of Windows SharePoint Services to solve a rudimentary Workflow problem to be the equivalent of 'using a cannon to kill a mosquito'. Now who could possibly argue with that?