A revolution every generation or two probably sweeps away the traditions of the ancien regime. Since England/Great Britain/UK has not had a revolution for more than 300 years (and the revolutionaries of 1688/89 were conservative aristocrats deeply attached to tradition) the constitutional cobwebs never got swept away.

Things just accumulated over the centuries and were woven into a tapestry of custom. For example the ceremonial inspection of the cellars is not a real security measure. It is just that as the authorities found Guy Fawkes and some barrels of gunpowder down there in 1605, it became traditional for people in strange historic costumes to always go and check the cellars before the State Opening of Parliament. No doubt they lovingly recreated the cellars after the old Palace of Westminster burnt down in the nineteenth century, so that the ceremony could go on unchanged.