1. Song Yifu is a professor at the Basic Medical College of Heilongjiang University of Chinese Medicine. Because his work may involve subject teaching and scientific research, he may not have enough time and energy to give public lectures.

2. In addition, because the theme of Song Yifu's lecture mainly focuses on the health care of the elderly, the target audience may also be limited to some extent, resulting in his lecture not receiving as much attention and participation as before.

However, these are only speculations, and the specific reasons may need further understanding and confirmation.