So what? We're talking about an age where party leaders were determined exclusively by MP's. In fact, with regards to the Tories, the more popular you were with the British public, the more people in the parliamentary party became suspicious of you. If personal popularity in the country alone was a determinant of who should be leader, then Rab Butler would have been PM instead of Macmillan and Douglas-Home.
Just because Powell was highly popular with the public in the period 1968-1972, this does not mean that he would automatically have won a leadership contest if one was held in that time frame. I've no doubt that Powell would have done well, as he was probably the main right-wing critic of Heath's leadership at that point, but to say that he would have been automatically elected on the basis of his standing in the country is to misunderstand how the Tory Party operated in that period. If fact, it may have actually slightly dented his prospects in any hypohetical leadership contest more than anything else.