William Penn attracted many people to his colony primarily by promoting religious freedom and tolerance. His Quaker beliefs emphasized the importance of freedom of conscience, which appealed to various religious groups facing persecution elsewhere, such as Quakers, Catholics, Jews, and others seeking a refuge from religious oppression.
While he did encourage investment in the colony to support its growth and development, the key factor in attracting settlers was his advertisement of the colony as a place of religious liberty and social equality. Penn's practices, such as offering attractive terms for land (including land grants) and promoting a peaceful coexistence with Indigenous peoples, also played significant roles.
Although slavery was not initially prohibited in Penn's colony, Penn's initial vision included the idea of equality and justice, which supported a mixed view on the matter. His promotion of fair treatment and human rights attracted people who valued those principles.
In summary, the primary way Penn attracted settlers was through advertising the religious freedom offered in Pennsylvania.