Question: "Why did Cubitt build the new wing?"  Context: "By 1847, the couple had found the palace too small for court life and their growing family, and consequently the new wing, designed by Edward Blore, was built by Thomas Cubitt, enclosing the central quadrangle. The large East Front, facing The Mall, is today the "public face" of Buckingham Palace, and contains the balcony from which the royal family acknowledge the crowds on momentous occasions and after the annual Trooping the Colour. The ballroom wing and a further suite of state rooms were also built in this period, designed by Nash's student Sir James Pennethorne."  Answer:
the couple had found the palace too small for court life and their growing family