Frankly, I don't believe Charles even needs an excuse. He is not snubbing the Chinese president and his wife but on the contrary, will be not only meeting and greeting and welcoming them to the UK, but also lunch with them and have his own private time of discussion.
Perhaps its all been synchronized to perfection with Charles and Camilla remaining at home "to rest"
rolleyes
therefore putting William and Kate in a prime position to be able to fully participate in the state banquet as Will and President Xi Jinping will not only share conversations over dinner but also be seen together and put emphasis on the newfound cooperation that both countries are working towards with United for Wildlife's aims and purposes.
Maybe its only me but actually I see William's involvement in this state visit as more of a priority than Charles'. Out of all of the British Royal Family (and that includes HM herself), only William so far has an actual working relationship with President Xi Jinping himself.