https://screenrant.com/the-crown-show-ending-explained/

The Crown season 6 represented the end of an era, highlighting Queen Elizabeth II's feelings of obsolescence, yet continuing to do her duty.

Why The Crown Season 6 Ends In 2005 Explained By Creator (https://screenrant.com/the-crown-season-6-end-why-peter-morgan/)

The Crown's creator Peter Morgan justifies his choice to end The Crown season 6 in 2005, rather than extend all the way up until present day.