That's why she needs to rest now and do what her doctors tell her to do when it comes to the back and the rest of the skeletal system. Osteoporosis, when it advances, make it such that even rolling over in bed and inadvertently moving a wrist the wrong way can cause a fracture. Or a twist of the neck the wrong way or sitting in an awkward position for too long. I know from experience. The Queen is a bit over 25 years older than I am and I've had to resort to using wheelchairs in places rather than walk due to osteoporosis.
Most likely, the Queen *could* have used a wheelchair and attended the Cenotaph memorial but can you imagine the uproar and the worry and the concern that would have garnered? The focus of the service would have been more on the Queen than the meaning of the service itself. HM wouldn't want that to happen. She is a woman that holds dear the importance of this day and it's meaning and she wouldn't detract from that.