Because democracy means that the people decide when the khilafah is not determined in this manner. The sahaba didn't go to the ballot boxes to cast a vote for who the next khalifa should be. Democracy is also an extremely flawed system, where an uneducated majority can overrule a learned minority and thus lead to imbalance and downfall. Monarchies, albeit un-Islamic, have been shown to be more stable than democracies. Khilafah incorporates what makes monarchies stable: the khalifa rules until death. So, hypothetically speaking, if there was a general election in Pakistan under what you understand to be a khilafah, would you want a Zardari ruling Pakistan until death? Of course not. Democracies leave room for correction by limiting terms, but then this makes those in power care less about the country and more about re-electability. As I said, democracy means that the layperson votes, regardless of how informed he or she is. Why do you think Bush got elected twice, despite much opposition from the academia, both political and scientific?