On a more practical level, there are a number of reasons why America isn't and can't be a theocracy:
1. While evangelical Christians form a substantial minority of the population, America's population is in no way a Christian population. What makes you think __your__ theocracy would be the one that ultimately prevailed?
2. That rigid wall of separation between church and state that many evangelicals decry has in fact kept us from the sectarian violence that characterizes other countries that mix church and state. Personally, I have no real interest in living in Northern Ireland, Iran or Pakistan. And even nominally Christian European nations like France and Germany that have official state churches without sectarian violence have churches that don't try to get involved in politics -- they take their taxpayer money and keep quiet about matters of state.
3. In America, religion thrives in part because of separation of church and state. Again, I would cite Western Europe: In England, France and Germany, all of which have state churches, approximately 3% of the population is regular churchgoers. In America, with a wall of separation, the population of regular churchgoers is about 60%. These results speak for themselves.
4. Churches, like other institutions, are run by fallible humans, and power tends to corrupt. Again, Europe provides the example: The corruption that inevitably comes from mixing church and state ultimately led Martin Luther to condemn both institutions and start a Reformation.
5. Finally, our Constitution forbids it. I realize the Constitution is not always perfect and it can be changed, but for Reasons 1-4 above I think this particular change would be a bad idea.

