This is because it is a higher level of learning and you have to prove that you are capable of being in this environment. By presenting good grades it shows your ability to be focused and self disciplined to take on several responsibilities that come with the knowledge that you will acquire.
You need to prove to the college that you will be ready to handle their rigorous academic schedule. College can be overwhelming, the classes are very fast paced and it's easy to fall behind. Good grades show your dedication to school and your education.