According to Heatstreet:
…because of concerns that tourists could be a target for jihadists carrying out a truck attack.
The popular 45-minute guard ceremony…will no longer take place on Saturdays. It will continue on Mondays, Wednesdays Fridays and Sundays, but only with extra police officers on duty and more streets surrounding the castle blocked to traffic.
The decision to scale back the event is a result of the two atrocities involving trucks in mainland Europe last year – first in Nice and then in Berlin.
Earlier this month the Queen’s guards at Buckingham Palace changed their ceremonial routines, also citing security concerns.