Queen Alexandra was following Queen Victoria's behavoir. I always thought it was very sellfish to "condemn" princesses to become old maids just to be with their mothers, especially considering that the mothers were expected to predecease their daughters by far. However, this practice has been part of history until recent times.
Queen Victoria wanted 3 of her daughters to remain single and was almost forced by the circumstances to marry princesses Helen and Louise; however, QV was determined not to allow princess Beatrix to marry; there was a great conflict when she announced her will to marry prince Henry of Battenberg and the Queen refused to give permission. After 8 months of psychological war, the Queen eventually agreed to let her daughter marry, on condition that Beatrix and her family would live with her forever. After 10 years Beatrix's husband was so bored to live with his strict mother in law that he asked to be sent to expeditions in Africa, where he died from malaria right away.
Empress Maria Theresa of Austria, whose daughters were indeed very beautiful princesses, would not allow them to marry unless for dynastic reasons, meaning that those who did not find royal husbands (minor princes were not even considered) had to become nuns (Archduchesses Maria Anna and Maria Elizabeth). Maria Theresa only allowed her favourite child, Archduchess Maria Christina, to marry for love with Albert, Prince of Saxony. Archduchess Maria Amalia, who was forced to marry the Duke of Parma (grandson of the Kings of France and Spain) instead of his love Prince Charles of Zweibrucken, decided not to contact her mother ever again. Prince Charles became an enemy of Maria Theresa and fought against Austria in the war on the Bavarian Succession.
More recently, Emperor Franz Josef of Austria and his son Crown Prince Rudolph wanted to force Archduchess Maria Valeria to marry either the King of Portugal or the Grand Duke of Tuscany; however, Empress Sissi came to the rescue of her favourite child; after a tremendous rift Sissi prevailed and Maria Valeria was allowed to marry for love with Archduke Franz Salvator of Austria, who was considered a minor prince and not a good match for an Imperial Archduchess.