At the time it was introduced there was a cumulative taxable income limit that applied to all businesses, so you were able to shelter income at the preferential tax rate up until you had reached a certain level of taxable income. You could regenerate access to that by paying dividends. As a business paid out dividends, it would restore its cumulative taxable income limit.
At the time, credit unions argued they had a lesser ability to pay out those dividends due to provincial regulatory requirements, so the cumulative limit put in for them was based on members' shares and deposits as a means of providing an equitable or equivalent access to the tax rate as it was structured at that time.