^I think it's because once Her Majesty shows up, it's back to being a state occasion. Not to mention that at her age, that would be a very long day - she may just flat not be up for all that.
I doubt those not invited to the official receptions are going to be left scrambling for a place to go after the ceremony. There will probably be dozens - if not hundreds - of private receptions held by various embassies, dignitaries, and officials for anyone who's invited to the ceremony.
I think the standards of etiquette for a royal wedding are more along the lines of those for a coronation or inauguration than would be applied to an ordinary couple.