The main reason is because credit unions are non-profit organizations desgined to serve and to provide financial assistance to its members and not to generate profit for its shareholders, as it is the case of banks, which are companies.
A credit union is a financial cooperative. Its members provide money to the union (as if they were investing in shares), and these funds enable to give loans, set deposit accounts, and provide other financial products and services to each other through the union. If any income is earned, it will be used to finance projects or services which are benefitial whole community.