Because kendo is based on the Japanese sword arts. It is an excellent vehicle for recognizing, creating, and taking advantage of openings in your opponent's defenses. This is what it was developed for.

Nakamura Taisaburo spoke of kendo as one leg of the tripod. He said that one had to do kendo, iaido, and tameshigiri to become a complete swordsman. I don't know that I entirely agree with him, but that's why kendo is still called a sword art.