One additional factor which I expect is significant is the number of Trump supporters who decided not to come due to the leftwing threats of violence (which turned out to be real and serious).
It’s one thing for a young/strong/adventurous person to risk getting caught in a violent protest, but far different for an older person or a middle-aged person with family responsibilities, or a person with physical limitations.
True, DJT supporters are scattered throughout the country and not concentrated in areas like DC, Baltimore, NY, Philly etc. All those cities are leftwing and within a 3 hr train ride to DC. not to mention all the Universities around that area. Easy to get leftist to rallies in DC vs middle America to come to DC. However, once DJT started his speech that place was packed with only small areas way in the back not filled. Many have posted pics on their blogs and twitter with over flow crowd. It was 2 to 3 hour delays to get through check points to reach the area to stand to watch the speech, people filtered in while the ceremony was going on ... it’s really a disgrace that the left created this situation then brag how nobody was there at the beginning of the speech. Gee, you talk about violence for weeks, create over the top security that cause people to be delayed hours getting into the mall area, and then take pics talking “nobody was there”.
The violence and rain would have turned a lot away.