You should not have to explain why you want a Bus Driver job. Rather, detail in your cover letter and resume those skills that relate to bus driving.
Visit any of the larger job boards, i.e., Monster.com, HotJobs.com, etc. and conduct a search for Bus Driver jobs. Note the language used in the job posting, and put that information in your resume and cover letter.
Use Free-Resume-Builder.net's (http://www.free-resume-builder.net) free online resume and career letter builder tool to build your Bus Driver resume and cover letter.