After Henry VII won the crown at the Battle of Bosworth, he had to do everything he could to strengthen his claim. Part of that was his marriage to Elizabeth of York on January 18, 1486. However, it was not as simple as just marrying her. She would need to give him heirs to the throne and be someone the English people supported.
At the time of winning the crown, Henry VII was already betrothed to the former princess. However, Richard III had done everything he could to discredit the children of his brother, Edward IV. Elizabeth Plantagenet had been made illegitimate, and therefore unable to become Queen of England in her own right. Henry VII went to work to put that right.