Why aren't all educational institutions non-profits?

Because to run any organisation we need money - capital to meet the short term and long term expenses. for non profit institutes this money is generally provided by the donors who donate the money as charity or some time funds by the central or  the state government.

But if this has not been managed the organisation should have to find the other source of income to general money to meet the expenses. 

So they can manage with fees they are getting from the students in the form of direct income.

Yes the dark side is that  that it became a profitable business now a days as most of the educational institutes targeting the profit and not the quality of education.

Because their founders are interested in money.