Why Prince Charles wasn't allowed to sit in Queen's throne for the State Opening of Parliament

ok.co.uk:

The Queen's throne was 'set aside' and remained empty today as she missed the State Opening of Parliament for the first time in over half a century.The monarch was absent from the lavish annual ceremony and did not read The Queen's Speech for the first time since 1963 due to "episodic mobility problems" she has been experiencing.

However, rather than the speech being read by the Lord Chancellor as has happened previously, in what has been described by commentators as a historic moment, Her Majesty instead delegated the responsibility to the heir to the throne, her eldest son Prince Charles.

The Queen used a legal instrument called Letters Patent to authorise her son to give the address on her behalf, and for Prince William to attend for the first time as one of her counsellors of state.

Sources said ahead of the event that the Prince of Wales was 'of course ready to support Her Majesty the Queen.' However, he was not permitted to sit on the royal throne in the House of Lords chamber where the speech is given and where members of the House of Commons are summoned to come and witness it.