> why do you need to acquire the non-voting shares to acquire the company?

Control doesn't alleviate the duties, fiduciary and of loyalty, the controlling person has to its shareholders. An acquirer who only bought the votes would still need to act in all shareholders' interests (or face litigation). To have a free hand, and full access to those cash flows, an acquirer thus must purchase the whole company.

because nonvoting shares still represent economic ownership of the company. for example, they get pro rata dividends if dividends are ever declared.