Then why didn't he base the constitution on Sharia Law?
Sharia wouldn't correspond to the constitution as it would the laws on the books, and there were aspects of Sharia law on the books.
The consitution, to my knowledge, was not formed by Jinnah or in his time.
Of the consitution, he wrote:
[quote]
The constitution of Pakistan has yet to be framed by the Pakistan Constituent Assembly. I do not know what the ultimate shape of this constitution is going to be, but I am sure that it will be of a democratic type, embodying the essential principle of Islam. Today, they are as applicable in actual life as they were 1,300 years ago. Islam and its idealism have taught us democracy. It has taught equality of man, justice and fairplay to everybody. We are the inheritors of these glorious traditions and are fully alive to our responsibilities and obligations as framers of the future constitution of Pakistan. In any case Pakistan is not going to be a theocratic State to be ruled by priests with a divine mission. We have many non-Muslims --Hindus, Christians, and Parsis --but they are all Pakistanis. They will enjoy the same rights and privileges as any other citizens and will play their rightful part in the affairs of Pakistan
[/quote]
Islam had a role to play in the state (i.e. not secular); it was not a state run by clerics (which he refered to as a theocracy). But then, I'm hard pressed to find a single empire/state/kingship/etc in the history of Islam that was run by clerics. Such comments were no doubt to appease his british masters (he was a Gov General, and so a subject of the Empire), who probably had no idea what he was talking about.
The latter part of his speech is often taken out of context to imply he advocated a secular pakistan, rather than convey his notion of the universality of islamic principles.