Why are people afraid to "speak ill of the dead?"?

I had a grandmother who died recently. She was a miserable person. I got into an argument with my brother because I was honest about my regards to her passing which was something along the lines of “good.” Everyone felt that way when she was still alive, but when she died, everyone pretended that they liked and loved her the whole time and that they were so sad that she was gone. All of a sudden everyone is depicting this person as a saint and I’m a horrible person for being honest. I feel that people shouldn’t say what they don’t mean and change it as soon as it’s convenient.