Because I think a funeral of someone close to you (if not close to Whitney at the time of her death, she was definitely close to Cissy) should take precedence over a concert. Makes sense to me. If she was able to do the concert that night, there is no reason she shouldn't have been able to make it to Whitney's funeral. She should have been there.
The video has been pulled, I tried to link it yesterday. When Aretha was on the Today Show she was evading answering what she would sing at the funeral. Then she went reflective - body language. Al Roker was left filling in the gap. She did read the poem, but it could be taken as derogatory if another Diva. It was Twinkle Twinkle Little Star, but a couple of words changed to make it about Whitney. I don't know their relationship, but I don't see a lot of coverage about them spending time together throughout the years, so maybe she didn't feel obligated. She also found out via television news. She is in her 70's though, and in poor health, so maybe it was just too much for her to deal with. We can guess till the cows come home and will not tell us the truth. There could be some bad blood. Maybe she just couldn't been around addicts and drugs, who knows.
As I so indelicately pointed out, I could not have sat through that lengthy funeral. I need potty breaks, lol! I would imagine medication times, meals, moving around would also be a factor. If she has been taking any cancer treatment (I think that was the news) she probably could not handle that and a show. Nor get back in time for a show.