Because when Tsunade was named as Hokage, Kakashi wasn't as strong as he is now. Tsunade was probably stronger
Also, there are certain things that are awaited for a kage, not only power. The kage is the one who is one of the strongest(not necessarly the strongest) in a village, has a great reputation all over the world, the one who is one of the most experimented, and the one who can protect his village efficiently. Tsunade was better for the job. She is much more experimented than Kakashi, respected as one of the 3legendary sannin, has a great medical ninjutsu what she can use to heal and protect everyone in the village thanks to Katsuyu which can split itself in multiple parts, and also when she was named as Kage, she was probably around Kakashis level or a bit stronger
Remember that even when Kakashi was about to be Kage, they hesitated by saying "he is too young", etc And kakashi himself said that Tsunade was better for the post. Jiraya himself who was about to be a kage said that Tsunade was better than him for the post, but that doesn't mean she was stronger than Jiraya, right?
The same goes for Bee for example. He is stronger than A, but he is not Kage