There is no reason why she could not have attended the service in a wheelchair. The reason she did not was she has memories of her own mother , HRH The Queen Mother, attending a function outside Buckingham Palace in a wheelchair, and swore she would never take to a chair. Pity as many people would have liked to see her there, I think.
Personally, I don't think she will last much longer. She has got the Jubilee over and have a feeling she is now ready to join Prince Philip, wherever he may be.