The answer is that the china cups served in the 17th and 18th centuries were so delicate that they cracked from the heat of the tea. Milk was added to cool the liquid and prevent the glasses from cracking. For this reason, it is said that even today many Brits add milk to their cup BEFORE adding it to the tea.
Today, though, one typically adds milk only to very bold teas, which tend to be or contain camellia senensis assamica AKA ‘Assam’ tea, which is the variety of the plant native to India, (and now found in Africa). Many blends of tea (such as English breakfast) use assamica tea to add the boldness of character to support the addition of milk.