Mattcutts explained the reason in the following video. Google usually gives more importance to fresh content than old content. Infact Google would like to test whether new content is better than old content or not. Google will rank each fresh article in the top positions for few keywords and look for the following factors.
- How the visitors are engaging with the content?
- How much time they are spending?
- How visitors are linking the content?
- How visitors are sharing the content in social platforms?
Based on above factors, Google will know the quality of old vs new content. If the new content is good and link worthy compared to old content then Google will make your ranking stable in top positions.