From a schools' information sheet issued by the Cathedral:
"St Paul’s is often nicknamed ‘the Nation’s Church’. Many National services are held here each year. These are often held to celebrate or remember special events in the country.
Drama, dance, debates and concerts often take place here too"
https://www.stpauls.co.uk/documents/Education/All%20About_the_Cathedral.pdf
So using chairs, instead of pews, will offer greater flexibility in the seating arrangements.
Further, if there will be a 'full house' for an important state occasion, it's better to have individual seats for each dignitary (rather than, as sometimes happens at weddings, fifteen people trying to squeeze into a pew intended for 10 people while there's still plenty of space a few pews further back). 'Reserved seats' are often needed for such occasions; it's far easier to achieve that with individual chairs, rather than long pews.
Lastly, the slight curvature of the seats (and their backs) will make them rather more comfortable than pews (as anyone who's ever sat through a lengthy performance of Handel's Messiah in a church can testify!).