Because they’ll get your story wrong.
It doesn’t matter who “they” are. No one can write your story the way you can. When I look at pictures of my grandparents, I realize that I can never fully know them just from my memories. I treasure all the letters, diaries, and other documents they left. Even with them, however, there are going to be gaps in my understanding.
This isn’t to say that your memoir will be entirely accurate. No one is the villain in her own story. Even if you fess up to the errors you made that you know about, the ones you didn’t recognize as errors will go down in someone else’s story. So present your side now. Regardless of the extent of your legacy, don’t you want your contributions remembered?
Luck and wisdom!