There's a very good chance that among the 30,000 or so attending the jamboree, there will be one or two carriers of Covid. I imagine that's the main reason the Palace is a little cautious.
The thing is NJ the Queen is like her mother was, a bit on the vain side. Still trying to walk in heeled shoes, not using a walking stick until it became essential, and I don't think the Queen is happy about wearing a mask, so it wouldn't surprise me if she has insisted she won't wear one, so they've banned her. :-)
The Queen is a 95-year-old woman trying to do the same job as she was at 25. Of course she should rest. Apart from anything else a meeting of the Greenies is hardly a state national ceremony. There is veritable plethora of adult royals who coud take her place if it is felt essential to have a royal presence. I think the prime concern now is that HM makes it to February 6th and her 70th Jubilee